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DRIVING PERFORMANCE & Cannabis studies


Concerns about the role of cannabis in collision causation date back many years, although much less is known about the impact of this drug on collisions than alcohol.

Among the reasons for this has been the much greater difficulty involved in measuring the presence and amount of cannabinoids compared to alcohol.

Video - Cannabis and Driving Research

Science & Research

2014 - Study ~ An examination of the validity of the standardized field sobriety test in detecting drug impairment using data from the drug evaluation and classification program.

2014 - Study ~ Driving under the influence of synthetic cannabinoids ("Spice"): a case series.

2013 - Study ~ Driving Under the Influence of Cannabis: Pitfalls, Validation, and Quality Control of a UPLC-MS/MS Method for the Quantification of Tetrahydrocannabinol in Oral Fluid
Collected With StatSure, Quantisal, or Certus Collector.

2013 - Study ~ Endocannabinoid system modulator use in everyday clinical practice in the UK and Spain.

2013 - Study ~ Impact of prolonged cannabinoid excretion in chronic daily cannabis smokers' blood on per se drugged driving laws.

2013 - Study ~ Driving under the influence of synthetic cannabinoids ("Spice"): a case series.

2013 - Study ~ Risk of severe driver injury by driving with psychoactive substances.

2013 - Study ~ Police custody following driving under the influence of cannabis: A prospective study.

2013 - Study ~ Comparison between self-report of cannabis use and toxicological detection of
THC/THCCOOH in blood and THC in oral fluid in drivers in a roadside survey.

2013 - Study ~ Blood Synthetic Cannabinoid Concentrations in Cases of Suspected Impaired Driving

2013 - Study ~ Prevalence of synthetic cannabinoids in blood samples from Norwegian drivers suspected of impaired driving during a seven weeks period.

2013 - Study ~ THCCOOH concentrations in whole blood: Are they useful in discriminating occasional from heavy smokers?

2013 - Study ~ Prevalence of alcohol and other drugs and the concentrations in blood of drivers killed in road traffic crashes in Sweden

2013 - Study ~ Cannabis use: a perspective in relation to the proposed UK drug-driving legislation.

2013 - Study ~ Analysis of AM-2201 and metabolites in a drugs and driving case

2013 - News ~ Imposition Of Per Se Limits For Drugs Don't Reduce Traffic Deaths

2013 - News ~ Michigan driver who uses medical marijuana wins appeal

2013 - News ~ Pot smell isn't cause to arrest everyone in a car

2013 - News ~ Medical Marijuana Laws Lead To Decrease In Alcohol-Related Deaths

2012 - Study ~ A placebo-controlled study to assess Standardized Field Sobriety Tests performance during alcohol and cannabis intoxication in heavy cannabis users and accuracy of point of collection testing devices for detecting THC in oral fluid.

2012 - News ~ Marijuana Users Are Safer Drivers Than Non-Marijuana Users, New Study Shows.

2012 - News ~ Reasons Why Marijuana Users Are Safe Drivers.

2012 - News ~ It Turns Out That Smoking Marijuana May Actually Make You A Safer Driver.

2012 - News ~ Cannabis and psychomotor performance: A rational review of the evidence and
implications for public policy

2012 - News ~ Marijuana Users Are Safer Drivers Than Non-Marijuana Users, New Study Shows

2012 - News ~ It Turns Out That Smoking Marijuana May Actually Make You A Safer Driver

2012 - News ~ Reasons Why Marijuana Users Are Safe Drivers

2012 - News ~ 7% of California Drivers Test Positive for Marijuana, but Are They Impaired?

2011 - Study ~ Medical Marijuana Laws, Traffic Fatalities, and Alcohol Consumption.

2011 - Study ~ The prevalence of cannabis-involved driving in California.

2011 - Study ~ Alcohol, psychoactive drugs and fatal road traffic accidents in Norway: a case-control study.

2011 - News ~ Study shows medical marijuana laws reduce traffic deaths.

2011 - News ~ Why Medical Marijuana Laws Reduce Traffic Deaths.

2011 - News ~ Colorado’s 5ng/ml per se DUID bill dies again as new research backs higher thresholds for regular users.

2010 - Study ~ Sex Differences in the Effects of Marijuana on Simulated Driving Performance.

2010 - Study ~ The effects of cannabis and alcohol on simulated arterial driving: Influences of driving experience and task demand.

2010 - News ~ Study: Marijuana Has Little Effect On Driving.

2010 - News ~ Hartford Hospital Studies Effects Of Marijuana Use On Driving Skills.

2010 - News ~ Psychomotor Impairing Effects Of Cannabis Are Nominal In Experienced Users, Study Says.

2009 - Study - The effect of cannabis compared with alcohol on driving.

2008 - Study ~ Effects of THC on driving performance, physiological state and subjective feelings relative to alcohol.

2008 - News - Cannabis and Driving: A Scientific and Rational Review.

2008 - NewsMarijuana and Driving Not So Dangerous After All.

2008 - News ~ Driving under the influence of cannabis: a 10-year study of age and gender differences in the concentrations of tetrahydrocannabinol in blood.

2007 - Study - Fitness to drive in spite (because) of THC.

2007- Study ~ Roadside sobriety tests and attitudes toward a regulated cannabis market.

2007 - Study ~ Developing limits for driving under cannabis.

2005 - News - Drivers With THC in their Blood Have Only a Small Increased Risk to Cause an Accident.

2001 - Study ~ Cannabis use and traffic accidents in a birth cohort of young adults.

2000 - Study ~ The influence of cannabis on driving.


1999 - Study - Marijuana And Actual Driving Performance.

1999 - News ~ University Of Toronto Study Shows Marijuana Not A Factor In Driving Accidents.

1998 - Study - Cannabis and driving.
1993 - Study ~ Marijuana And Actual Driving Performance.

1992 - Study ~ The Incidence and Role of Drugs in Fatally Injured Drivers - DOT HS 808 065.

1982 - 1998 - Study - Abstracts of several studies.

1976 - Study ~ Simulated Flying Performance After Marihuana Intoxication.

1976 - Study ~ Marijuana effects on simulated flying ability.

1969 - StudyComparison of the Effects of Marijuana and Alcohol on Simulated Driving Performance.

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Cannabis and driving

The influence of cannabis on driving
Prepared for Road Safety Division, Department of the Environment, Transport and the Regions
B F Sexton, R J Tunbridge, N Brook-Carter (TRL Limited), P G Jackson (DETR),
K Wright (University of Birmingham), M M Stark (St George's Hospital
Medical School) and K Englehart (Principal Police Surgeon)

Recreational Drugs and Driving Prevalence Survey:
Dave Ingram, Becki Lancaster and Steven Hope
Development Department Research Programme
Research Findings No. 102
System Three Social Research
Qualitative Study: Joanne Neale, Neil McKeganey, Gordon Hay
Centre for Drug Misuse Research, University of Glasgow
John Oliver Department of Forensic Medicine, University of Glasgow

Cannabis intoxication and fatal road crashes in France: population based case-control study
Bernard Laumon, Blandine Gadegbeku, Jean-Louis Martin, Marie-Berthe Biecheler
British Medical Journal  2005;331:1371
10th December 2005

Marijuana And Actual Driving Performance
Conducted on behalf of: U.S. Department of Transportation,
National Highway Traffic Safety Administration
(DOT HS 808 078), Final Report, November 1993
HWJ Robbe Institute for Human Psychopharmacology,
University of Maastricht, P.O. Box 616, NL-6200 MD,<
Maastricht, The Netherlands

Cannabis and road safety:
Dr G.B. Chesher
Department of Pharmacology University of Sydney and National Drug and Alcohol Research Centre University of New South Wales.
An outline of research studies to examine the effects of cannabis on driving skills and actual driving performance

Drugs and Accident Risk in Fatally-Injured Drivers
Olaf H. Drummer, Ph.D.
Victorian Institute of Forensic Pathology, Department of Forensic Medicine, Monash University,
57-83 Kavanagh Street, South Melbourne 3205, Australia

Drugs and Driving
Alcohol and Drugs Foundation of Australia


Cannabis and driving
Cannabis and driving - more evidence
Australian Press reports

The AGE 21 October 1998 pA5
CANBERRA TIMES 21 October 1998 p4

The largest study ever done linking road accidents with drugs and alcohol has found drivers with cannabis in their blood were no more at risk than those who were drug-free. In fact, the findings by a pharmacology team from the University of Adelaide and Transport SA showed drivers who had smoked marijuana were marginally less likely to have an accident than those who were drug-free. A study spokesman, Dr Jason White, said the difference was not great enough to be statistically significant but could be explained by anecdotal evidence that marijuana smokers were more cautious and drove more slowly because of altered time perception.

The study of 2,500 accidents, which matched the blood alcohol levels of injured drivers with details from police reports, found drug-free drivers caused the accidents in 53.5 per cent of cases. Injured drivers with a blood-alcohol concentration of more than 0.05 per cent were culpable in nearly 90 per cent of accidents they were involved in. Drivers with cannabis in their blood were less likely to cause an accident, with a culpability rate of 50.6 per cent. The study has policy implications for those who argue drug detection should be anew focus for road safety. Dr White said the study showed the importance of concentrating efforts on alcohol rather than other drugs.


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Dr G.B. Chesher

Department of Pharmacology University of Sydney and National Drug and Alcohol Research Centre University of New South Wales.

Dr Chesher provides an extensive coverage of the latest Australian and overseas research on the impairing effects on driving of cannabis, particularly relative to those of alcohol.


1. Executive summary

2. Introduction

  • 2.1 Pharmacology and pharmacokinetics
  • 2.2 Behavioural pharmacology and psychology

3. Studies using the techniques of epidemiology

  • 3.1 Pharmacology and pharmacokinetics
  • 3.2 Pharmacokinetics

4. A comparison of the effects of alcohol and cannabis on skill performance and driving skills

  • 4.1 Laboratory tests
  • 4.2 Duration of cannabis-induced impairment in laboratory tests
  • 4.3 The effect on laboratory tasks of alcohol and cannabis in combination
  • 4.4 Driving simulators
  • 4.5 On-road driving

5. Epidemiology

  • 5.1 Questionnaire based surveys
  • 5.2 Incidence of drug detection in crash involved drivers
  • 5.3 Attempts to assess whether or not the driver who has detectable drugs in the blood stream was culpable in the accident

6. The use of 'Responsibility Analysis' or estimation of 'culpability' to determine the role of drugs in crashes

7. Alcohol and cannabis in epidemiological studies

8. Summary (of the evidence presented above)

There is no doubt that cannabis, smoked or taken by mouth produces a dose-related deficit in tests of performance skills as conducted in a laboratory.
Using driving simulators and on-road real vehicles, cannabis has been shown to affect driving performance. However, the effects are less severe than would be anticipated from the evidence obtained from the laboratory studies of individual tests of skills performance.

A description is given of epidemiological studies to determine the role of cannabis in road crashes. The pharmacological problems associated with these studies are described. The results of studies within the last 10 years have failed to present clear evidence for a role of cannabis in road crashes. The role of alcohol in all studies has proved to be dominant.

The evidence indicates that there is a clear difference in the mode of action of cannabis and alcohol, both pharmacological and behavioural and this is presented and the implications described.
The most recent of studies of cannabis and driving (Robbe & O'Hanlon, 1993), which was sponsored by the U.S. National Highway Safety Traffic Administration included a review of the literature. The authors' comments in summary of their literature review and of their own results include the following:

The foremost impression one gains from reviewing the literature is that no clear relationship has ever been demonstrated between marijuana smoking and either seriously impaired driving performance or the risk of accident involvement.

The epidemiological evidence, as limited as it is, shows that the combination of THC and alcohol is over-represented in injured and dead drivers and more so in those who actually caused the accidents to there is little if any evidence to indicate that drivers who have used marijuana alone are any more likely to cause serious accidents than drug free drivers.

Of the many psychotropic drugs, licit and illicit, that are available and used by people who subsequently drive, marijuana may well be among the least harmful. Campaigns to discourage the use of marijuana by drivers are certainly warranted. But concentrating a campaign on marijuana alone may not be in proportion to the safety problem it causes.
In this paper I will examine briefly the studies which have sought an understanding of the effect of cannabis and of alcohol on driving skills and their role in road crashes. This information has been based upon scientific data which have been collected from several scientific disciplines. I have outlined these in earlier papers and will only mention them briefly here.

The major purpose of this paper is to compare the two drugs, alcohol and cannabis and the status of the evidence as to their role in road crashes.

The determination of the legal limit for alcohol has been achieved in a scientific manner. There are pharmacological reasons why it has not been possible to follow these same techniques with drugs other than alcohol, including cannabis. This paper will draw attention to these problems.

First, we might briefly outline the nature of the evidence which has been generated to examine the effects of cannabis on driving skills and as a causative factor in road crashes. This information has been derived from the employment of three scientific disciplines:

2.1 Pharmacology and pharmacokinetics

Pharmacology is the study of the way a drug exerts its action in the body. This involves an understanding of the sites and the body systems where the drug acts and the consequences of this drug-system interaction. Information obtained from these studies can help to formulate an hypothesis as to how the drug may influence driving behaviour.

The pharmacological discipline known as pharmacokinetics studies the fate of the drug after it has been taken. It provides information as to the rate of absorption from the site of administration; the manner of its distribution in the body up to the delivery to its site of action (eg. the brain). Pharmacokinetics also studies the way the body eliminates the drug from the body and includes the understanding of the metabolism and excretion of the drug.
2..2 Behavioural pharmacology and psychology

These involve studies of the effects of the drug on human behaviour. The behaviour of relevance to this discussion concerns those skills which are (or are related to) those necessary for the safe control of a motor vehicle or other items of machinery. Psychological studies also involve the effects of the drug on mood and cognition.

The three classifications of these studies are:
(i) Those performed on specific tests of behaviour or psychological functioning (for example, tests of reaction times of various degrees of complexity; tracking; divided attention or vigilance);
(ii) Those performed in a driving simulator; and
(iii) Those performed in a real car, either in a closed course or in real traffic.


These studies aim to determine whether or not a causal relationship between drug use and a motor vehicle crash exists.

I shall look at each of the above factors and will compare the two drugs alcohol and cannabis in the light of current evidence. In interests of time and space I have in this summary referred to reviews of the literature and have made only a brief description of the studies themselves. A fuller description of these can of course be sourced from the original literature of the cited reviews.
3.1 Pharmacology

First, the drugs themselves. With the increase in pharmacological knowledge it is known that most drugs act upon specific receptors. A receptor is a specific site in tissues, frequently on the cell membrane, which has a specific structural affinity (shape) for a naturally occurring molecule.

The interaction between receptor and the endogenous molecule is part of the body's normal, physiological functioning. Most drugs exert their activity by acting upon these receptors. Examples of such drug-receptor interactions are the opioids (morphine etc) and the opioid receptors; the antihistamines and the histamine receptors and the benzodiazepines which act on the benzodiazepine receptors. The endogenous substances that physiologically act on these receptors are, respectively, the endorphins and enkephalins on the opioid receptors; histamine on the histamine receptors; however the identification of the physiological substance for the benzodiazepine receptor has yet to be identified.

Research within the last five years has revealed that the cannabinoids, such as delta-9-tetrahydrocannabinol (THC) from the cannabis plant exert their effects on specific receptors known as the cannabinoid receptors. To date two cannabinoid receptors have been described and an endogenous (physiological) substance has been identified. This has been given the name 'anandamide'. It is very likely that in the near future more cannabinoid receptors will be described and more endogenous substances that act on these receptors will be identified. An historical overview of these findings has recently been published.

In contrast, the evidence strongly indicates that the drug alcohol does not act on a specific receptor, but acts more widely in a non-specific manner on the cell membranes themselves. This understanding is supported by the evidence that alcohol exerts effects on most of the tissues of the body and in excess is toxic to most tissues. The reader is referred to a recent review on this subject by Dufor and Caces.

Drugs which act upon a specific receptor produce their effects in doses measured usually as nanograms or micrograms per kilogram of body weight. Alcohol doses are measured in grams per kilogram - many hundreds of thousands times greater than those of most other drugs. Alcohol is a very non-specific drug.

Another important factor is that receptor-specific drugs exert their activity only on those cells which bear the specific receptor. In the case of the cannabinoids these receptors are found only in the brain in the basal ganglia, the cerebellum, the brain stem, thalamic nuclei, hypothalamus and corpus callosum. On the other hand alcohol affects all nerve cells to which it is delivered by the circulating blood.

Consequently it is not surprising that differences in the action of alcohol and the cannabinoids have been described in their effects on mood and behaviour. These will be discussed below.

3.2 Pharmacokinetics
The pharmacokinetics of alcohol and the cannabinoids could hardly be more different.
The apparent volume of distribution of alcohol (the volume of fluid in which the drug seems to be dissolved throughout the body) is quite low, consisting of the 41 litres of body water, providing a value of about 0.59 litres/kg. Cannabinoids, on the other hand, are very fat soluble and have a high volume of distribution which has been estimated to be about 10 litres/kg.

The meaning of these values is that the concentration of alcohol in the blood provides a reliable estimate of the concentration of the drug in the brain. This in turn provides a reliable estimate of the degree of impairment of the drinker. In addition to this, alcohol is excreted via the lungs to the breath and the blood : breath ratio is such that the determination of the alcohol in breath provides a reliable estimate of the blood alcohol concentration. It is because of these pharmacokinetic properties of alcohol that it has been possible to accumulate the epidemiological data upon which our drink-driving laws have been based.

Cannabinoids, on the other hand are lipophilic (fat loving) and are distributed in the fatty tissues of the body. When smoked, which is the most common route of administration, the cannabinoids are rapidly absorbed from the lungs into the bloodstream. Being so fat soluble the cannabinoids readily cross membranes, leave the circulation and are rapidly 'dumped' into various tissues of the body, including the brain. In this way the concentration of cannabinoid in the blood declines very rapidly as indicated in Fig 1.

As indicated in the Figure, we can describe the concentration of cannabinoid across time in the blood in the three phases: absorption, re-distribution and elimination. The steep upward curve of THC represents the inhaled THC being absorbed into the blood through the lungs; the equally sudden drop in the concentration of THC represents the drug being 'dumped' from the bloodstream into fatty tissues.

This redistribution phase 'flattens' out as the 'dumped' THC re-enters the blood and is then metabolised in the liver-the elimination phase. It is important to note that the sudden decline in the concentration of THC (the psychologically active cannabinoid) in the blood does not represent drug metabolism but rather the rapid re-distribution of the drug from the blood into other tissues. The metabolism of the cannabinoids takes place when these 'dumped' cannabinoids are released back into the bloodstream whence they pass through the liver and are very rapidly metabolised and subsequently excreted.

Figure 1. The blood concentration of THC (squares) and its inactive metabolite, carboxy THC (THC Acid; diamonds) after the smoking of a marijuana cigarette. Each point is the mean of results from six volunteers, all of whom were free from cannabinoids before smoking the drug. [The 925±7mg refers to the average weight of the cigarettes and the 1.32% refers to the dry weight concentration of THC]

Figure 1 also shows the blood picture of the inactive metabolite, carboxy THC (or THC acid). It is important to note several points about the pharmacokinetics of this substance.

First, in the study indicated here (Fig 1) all of the volunteers had no cannabinoids in their blood before they began smoking.

Second, the THC acid is formed in the liver from the metabolism of THC, therefore its appearance in blood follows that of the parent, THC.

Third, the THC acid concentration then increases and surpasses that of the parent molecule in the blood. At a time when the parent THC is in the blood at only a very low concentration, that of the metabolite is higher and exists in the blood for a longer time. Therefore, should the smoker smoke again before the parent molecule and its metabolite have been eliminated, the ratio of the concentrations of THC and of the THC acid will be different from that shown in Figure 1. This is because there will exist a higher concentration of the metabolite than of the THC in blood at the time when the next dose of cannabis is smoked.

For this reason, analytical data that provides a value only for the metabolite can only be validly interpreted as indicating recent consumption of cannabis; however the time of this consumption could be a matter of hours or days. For this reason the quantitative determination of only the metabolite is of no value to determine possible impairment.

To assess possible impairment the analyst must provide data for the active molecule, THC. And when this occurs, the only interpretation possible on present knowledge is to infer the recent consumption of the drug by smoking. To date no meaningful correlation between blood concentration of THC and impairment in laboratory tasks has been established. This point will be clarified when the results of the recent epidemiological studies are discussed below.

Yet another problem arises in the interpretation of blood concentrations of cannabinoids. The pharmacokinetics of the cannabinoids are quite different when the drug is taken by mouth. Space in this discussion precludes further discussion of the pharmacokinetics after oral administration, but suffice to say the absorption of cannabinoids taken orally is slow and erratic. The absorbed THC passes through the liver and is rapidly metabolised.

This results in a different proportion of THC to the metabolite, THC acid than encountered after smoking. There is a greater amount of entero-hepatic 'recycling' as some of the cannabinoids are stored in the bile in the gall bladder. These cannabinoids can later be 'recycled' and reabsorbed into the bloodstream when the gall bladder empties. In this country, most who use cannabis, smoke it.

It is also important to note that the detection of cannabinoids in a urine sample provide evidence only that the donor of that urine has been exposed to cannabis at some time in the past. It gives no indication at all of impairment or of intoxication. A frequent, heavy cannabis user may be excreting cannabinoids in urine for some weeks or in some cases, for more than a month. Those who take the drug by mouth also will be excreting the drug for a longer period.

4.1 Laboratory tests

Laboratory tests isolate specific psychological functions and determine the skill of the test subject on that function. Most studies test each volunteer on each test before and after taking the drug. For testing alcohol and cannabis, the choice of these tests rests upon an assessment of their relationship to the task of driving a motor vehicle. However, the fact is that no battery of separate tests comprehensively defines the actual task of driving. In fact, Joscelyn and others (Joscelyn et al., 1980) examined the plethora of methods employed in these studies and commented:

... many tests routinely employed have limited validity or no demonstrable relation to real-world driving. Measuring the 'same' behaviors often differ, raising questions about the comparability of experimental findings.

Laboratory tests, nevertheless do provide a 'screening' of the potential for drugs to impair specific behaviours. However, results from such laboratory testing should not form the sole basis for any judgement of the potential of a drug to impair actual driving skills or to increase the probability of an accident. For this reason, evidence for the traffic hazard associated with any drug should be confirmed by studies of actual driving (either using driving simulators or a real car) and by studies using epidemiological methods.

The data from laboratory testing of alcohol has been reviewed by Moskowitz and Austin and of the effects of cannabis by Klonoff, Moskowitz, and by Chesher. It is clear that both alcohol and cannabis cause dose-dependent deficits in the performance of specific laboratory tasks.

It is to be noted that the doses of cannabinoids in these tests are lower than those in use by many smokers of cannabis today. However, they may have been appropriate to the cannabis experience of the volunteers when these studies were conducted. In many of these studies, the volunteers were asked to rate the effect of the dose given with that of their general experience with the drug. In many (but not all) cases the doses given produced subjective effects which were as great as those generally experienced by the volunteers in their social use of the drug.

Looking at the Australian studies across time, from the 1970s to the 1990s these observations are in accord with the results expressed in a recent publication concerning the patterns of cannabis use in Australia. The earlier studies produced deficits in testing which were greater than those in the later studies. The data presented by Donnelly and Hall (1994) indicate that:

The prevalence of cannabis use seems to have been very low by contemporary standards in the early 1970s. It increased substantially throughout the 1970s and 1980s, levelled off in the late 1980s, and has probably shown a small increase in the early 1990s.

The phenomenon of tolerance to cannabis is well established and this in turn is a serious confounding variable in the studies with this drug. Tolerance develops with the regular and frequent use. This in turn depends upon the pattern of use of those in the study sample. The correlation of performance : dose : and tolerance requires further study.

There is very little information available as to the change in doses used across the years since the 1970s as most data refer only to frequency of use. Studies involving high doses of cannabis should be undertaken, but with due consideration given to the degree of tolerance of the volunteers to be studied.

The Australian data presented by Donnelly and Hall indicate that:

Most cannabis use is infrequent and intermittent, with about three-quarters of adult women and two-thirds of adult men having discontinued their use, or continued to use less often than weekly. The proportion of users who are weekly users is highest in the younger age groups. Rates of weekly and lifetime use are highest among those aged 20 to 24 years, and decline markedly with increasing age.

4.2 Duration of cannabis-induced impairment in laboratory tests.

Most studies have reported a duration of cannabis-induced impairment of the order of 4 hours. On the other hand there have been three studies which have reported a longer duration of cannabis effects of between 10 to 24 hours. However, these reports have been questioned for methodological or reasons of interpretation.

That of Yesavage et al. did not include a control group. Subsequently the study was repeated by Leirer et al. in an attempt to replicate this effect using a control group but was only able to show an effect up to four hours after smoking (ie. that described in the many other studies of this effect). A third study, also with a control group, did demonstrate an effect at 24 hours after smoking. The statistical significance of the effect required a statistical procedure (one tail 't' test) which is of questionable validity when there was no previous statistical proof that the effect was expected. This means that the effect was at best, only marginally significant. The study by Moskowitz et al, as described in Moskowitz's 1985 review (Moskowitz, 1985) was of a:

.... compensatory tracking task performed while simultaneously executing a visual search task as well as a critical tracking task. Performance was significantly impaired on the compensatory tracking task for more than 2 hours and upon the critical tracking task for up to 10 hours, albeit, intermittently during the period from 4 hours on. [emphasis added]

At present I think it is fair to conclude that the evidence for the long duration of cannabis induced impairment requires more study to confirm its validity. Furthermore, both tasks in which it was described are very difficult tasks. It has been argued that the use of cannabis by pilots in the 24 hours preceding flying may be more an indicator of poor judgement rather than a cause for concern about the residual psychomotor effects of cannabis.  

4.3 The effect on laboratory tasks of alcohol and cannabis in combination
The effect of this drug combination has been reviewed and only an outline will be given here.

There is very clear evidence from numerous studies of the effect of alcohol and of cannabis on the performance of specific tasks in the laboratory. Both drugs produce a dose related impairment on these tasks and the effect of the drugs when given in combination is essentially additive. Although of more academic than practical interest is the evidence as to the nature of this additivity.

Several studies have observed a trend that the effect of cannabis plus alcohol is less than additive, meaning that 1 + 1 is less than 2. In the most recent study, Dauncey et al. reported this effect, found to be statistically significant, and termed it to be a 'de-intensification'. In the light of the present knowledge of the quite different mode of action of cannabis and alcohol such an interaction is not necessarily surprising.

What is quite surprising and important however, is the result of a study by Perez-Reyes.

For pharmacological reasons the researchers studying the alcohol-cannabis interaction administered the drugs such that the peak of blood concentration of both drugs occurred as near as possible at the same time. Such is the thinking of the pharmacologist! Indeed Perez-Reyes and his colleagues had reported such a study showing an additive decremental effect of the drug combination. Interestingly, in their later study they had the volunteers smoke marijuana (placebo; 1.7% and 3.58% THC) before they commenced drinking alcohol (0.85g/kg) over a period of 30 mins.

This would have produced a BAC of the order of 0.1g%. Their results showed a dose-dependent effect for cannabis and the characteristic effects expected for the one dose of alcohol. However, no significant interaction between the two drugs was recorded. The authors concluded:

The lack of interactive effects, particularly on psychomotor performance, highlights the influence that the order of administration of the companion drug has on its interaction with the reference drug.

4.4 Driving simulators

A driving simulator is also a laboratory based apparatus. It is important to realise that it is only a simulation of real life driving and driving simulators vary greatly in the degree to which they can simulate the real event. It is fair to say that all but the most sophisticated and extremely expensive simulators are to the test subject, still a laboratory piece of equipment. They lack realism both in the dynamics of car driving and in the visual presentation of the road and other traffic. Nevertheless they are able to present simulated dangerous presentations to which the driver must respond. The effects of cannabis on performance in a driving simulator have been reviewed and a summary only is given here.

The early driving simulator studies, for the driver, were not interactive with the 'driving scenery' which was generally a film of the road to be covered and the driver had little or no control over the presented imagery.ÊÊ These studies showed no significant effects of marijuana on car control. However marijuana did produce the following effects, namely:

(a) An increase in decision latency before starting, stopping or overtaking;
(b) Impaired monitoring of a speedometer; and
(c) Reduced risk-taking behaviour in tasks requiring a decision to overtake a vehicle in the presence of an oncoming car.

Later simulator studies with apparatus with a more realistic driving dynamics and an interaction between 'scenery' and the driving manoeuvres did show marijuana effects on car control. The study by Smiley et al. found that cannabis increased lateral position variability, headway variability, and caused the 'driver' to miss more signs that indicated the need to follow another route.

On the other hand, cannabis caused the subjects to drive in a more conservative manner inasmuch as they maintained a longer headway when car following, refused more opportunities to overtake a vehicle in front and when they accepted this opportunity, they began to do so at a greater distance from the approaching vehicle. The effects of alcohol (at about 0.08g% BAC) in this study were surprisingly small.

Another and very similar study by Stein et al. showed alcohol effects were as one would expect and significantly affected practically every performance parameter. Alcohol (at about 0.1g% BAC) was associated with significantly increased 'accidents' (hitting obstacles or exceeding road edges by a full car width) and 'traffic tickets' (exceeding speed limit by 32 'radar checks'). Alcohol was also associated with increased lane deviations, speed variability, response times to signs, and errors in sign recognition. In contrast, cannabis was associated with few changes.

The mean speed travelled was lower and two measures of steering control changed significantly. Alcohol and cannabis in combination were associated with more adverse reactions than alcohol alone. Alcohol was consumed first and the performance testing was begun 15 minutes after the end of cannabis smoking.

4.5 On-road driving

Driving studies with a real car, conducted in an open field, of course present a more realistic experience of a motor vehicle than do simulators. However they usually require the driver to undertake manoeuvres that are not necessarily part of normal driving - such as weaving between cones. Those studies undertaken in on-road traffic naturally require great care on the part of the experimenter to avoid dangerous driving.

Therefore these studies are restricted in the measures that can be realistically taken. They are somewhat akin, for both the experimenter and the test driver, to a driver undertaking a test for a driving licence. Indeed, experimental studies of the effects of drugs using in-car performance have been described by Smiley as being really a simulation of real driving.

On-road driving studies vary considerably in their experimental design and in the tests of driving employed. In this paper, only the broadest outline of the results is given in the interests of brevity. Reviews of these studies have been presented and published. The reader is referred to the original studies or to the cited reviews for more information.

There have been to date, seven on-road studies to examine the effects of cannabis on driving performance. Each of these is outlined below:

1.  Klonoff studied volunteers in a closed course as well as in-traffic on a city road. The closed course study comprised eight tests and the response scores rested essentially on the number of cones struck. Testing was conducted in 4 blocks, each of 5 trials. The first three were taken as practice and the fourth, after drug treatment, were the test trials. The anticipated scores in the fourth block were determined by regression analysis on the assumption that the rate of learning or performance would continue at the same rate. Using this technique the author concluded that there was an impairment under cannabis. While the mean of the impairment was not large, the trend was clear.

 The city traffic study was conducted rather in the manner of a driving test by a driving examiner. The subjects drove for about 45 minutes on a course of 16.8 miles after being given their dose of cannabis. A strong trend towards impaired performance was indicated by the lower scores given by the examiner on judgement and concentration after the higher dose of cannabis.

2.  Hansteen et al. conducted a closed course study in which subjects were required to drive six times around a 1.1 mile course set out on an airfield. The course was set out with cones and poles and the number of these hit were counted. The course involved curves and straight sections and drivers were required to undertake various manoeuvres. The mean number of struck objects per lap increased from a mean of 13.2 in the placebo condition, 13.4 in the low cannabis dose, 16.8 for the high cannabis and 17.4 for the alcohol dose (BAC 0.07g%). The effects for the high cannabis dose and the alcohol dose achieved significance.

3. Casswell conducted a closed course study in which the behaviours sampled were more typical of those for real driving, than for the studies outlined above. Driving behaviours recorded included overtaking, responding to road signs, making a hairpin turn and driving through a narrow gap. A subsidiary reaction time task was also included to monitor attention. Driving behaviour under cannabis, alcohol and the combination was tested. After alcohol, and alcohol plus cannabis, the subjects showed poorer tracking performance and drove at increased speed over various segments of the course, including the hairpin bend, and the straight section. Under alcohol alone, the speed through the narrow gap was also increased.

 On the other hand, marijuana alone was not accompanied by steering or tracking errors. The mean speed dropped significantly after cannabis, both on the hairpin bend and on the straight section of the course.

Casswell suggested that drivers under the influence of cannabis appeared to compensate for what they perceived as being an adverse effect on driving. Compensation was exhibited by driving more slowly. This contrasted with the effects of alcohol. The increased reaction times to the subsidiary task under cannabis suggests an effect on attention. The extent of this effect was of the same order as that measured by the author in another study after 8 hours of continuous driving.

4. Attwood conducted a study on a closed course constructed on an airfield and, like Casswell, used measures appropriate to real driving including acceleration, following a lead car which varied its speed and responding to 'traffic signals'. The drug effects (alcohol, and two doses of cannabis alone and together with alcohol) recorded were not particularly robust, even with a complicated multivariate analysis which did distinguish the treatment conditions from each other.

5. The study by Peck and colleagues (Peck et al., 1986) from the California Department of Motor Vehicles, is best summarised by the authors' own summary.

Approximately 80 volunteer male marijuana and alcohol users received one of four experimental treatments: (1) marijuana, (2) alcohol, (3) marijuana and alcohol, or (4) double placebo.

After consumption, each subject drove a vehicle over a test course which simulated a number of real-world driving conditions.

Four post-drug runs were involved, separated by one hour intervals. The subject's performance was rated by an in-car examiner, outside observers, and computerised vehicle measurements.

Blood and urine specimens were extracted after each run to establish levels of tetrahydrocannabinol (THC), serum carboxy, and alcohol. A variety of multivariate statistical techniques were applied in evaluating treatment effects.

Both marijuana and alcohol had significant effects on driving performance, and the effects were particularly detrimental under the both-drugs treatment. The effects of marijuana were more rapid than those of alcohol and somewhat less severe for most tasks.
 In this study cannabis was smoked after the consumption of the alcohol dose. In discussing their results and comparing them with other studies, they had this to say:

There is a vast amount of empirical evidence documenting the effects of marijuana on a wide array of human performance measures-cognitive, psychomotor and affective. Although the literature has clearly established that marijuana affects all three domains and results in detriments in the ability to perform many psychomotor and cognitive tasks, the evidence is somewhat more equivocal on the question of actual driving skill and even more equivocal on the question of those aspects of driving skill that are related to safety and accident avoidance. [Emphasis that of Peck et al.]

6. Smiley et al. tested the effects of cannabis (placebo and two doses) and alcohol (placebo and BAC of 0.05 g%) in combination and the effect of alcohol alone (BAC 0.08g%) on driving in a closed course study using an instrumented car.

 The high dose of cannabis significantly increased headway and headway variability (ie distance from a car in front). Alcohol alone at the BAC 0.05g% produced an increase in speed, both in the straight sections of the road and in curves. In her review of her own study, and those of others, Smiley (Smiley, 1986) concluded:

In conclusion, marijuana does appear to impair driving behaviour. However, this impairment is mediated in that subjects under marijuana treatment appear to perceive that they are indeed impaired. When they can compensate, they do, for example, by not overtaking, by slowing down and by focussing their attention when they know a response will be required. Unfortunately, such compensation is not possible where events are unexpected or where continuous attention is required. Effects on driving behaviour are present shortly after smoking but do not continue for extended periods. [emphasis added]

7. The most recent and most comprehensive study of the effect of cannabis on driving on city roads and a public highway is that conducted in The Netherlands and was sponsored by the U.S. National Highway Safety Traffic Administration. An intelligent departure in methodology in this study from the others reviewed here is that the dose of cannabis used was determined in a pilot study using the volunteers who were to take part in the main study.

The aim was to estimate the dose these volunteers generally use on a social occasion. Accordingly socially appropriate doses (for these subjects) were chosen for the driving study. Three driving studies were then performed. The first was conducted on a closed section of a public highway with no traffic; the second on a highway with traffic and the third in city traffic.

The measure they have found to be of significance is the standard deviation of lateral position on the roadway (SDLP). It is a measure of the 'automatic' function of information processing in the driving task. Cannabis, in all tests produced a dose-related increase in the SDLP. Mean speed was somewhat reduced under cannabis as was the headway distance from the lead vehicle in the test in highway traffic.

The test under city driving conditions was conducted under one dose of cannabis and as a comparison, subjects were also tested under alcohol at a BAC of 0.04g%. Results in this test showed that this modest dose of alcohol, but not cannabis, produced a significant impairment of driving performance relative to placebo. Alcohol impaired driving performance but subjects did not perceive it. Cannabis did not impair driving performance yet the subjects thought it had. After alcohol, there was a tendency towards faster driving and after cannabis, slower.

This research group has conducted many studies with the same methodology and has accumulated much data on the effects of other drugs.

They therefore were able to indicate the extent of the impairment on the measure of SDLP. The greatest effects of cannabis in this study were 3.7 and 2.9cm. In other studies drugs, for example diazepam (Valium), or lorazepam (Ativan), produced increases of 7 and 10cm respectively. The authors commented:

In so far as its effects on SDLP are concerned THC was just another moderately impairing drug.
The authors go on to say that the effects of cannabis differ qualitatively from those of other depressant drugs, especially alcohol:

Very importantly our city driving study showed that drivers who drank alcohol overestimated their performance quality whereas those who smoked marijuana underestimated it. Perhaps as a consequence, the former invested no special effort for accomplishing the task whereas the latter did, and successfully. This evidence strongly suggests that alcohol encourages risky driving whereas THC encourages greater caution, at least in experiments.

Finally, Robbe contrasted the effects of cannabis when measured with laboratory based, individual tests in the laboratory, with those conducted in an on-road vehicle:

The results of these studies corroborate those of previous driving simulator and closed-course tests by indicating that THC in single inhaled doses up to 300 µg/kg has significant, yet not dramatic, dose-related impairing effects on driving performance.

They contrast with results from many laboratory tests, reviewed by Moskowitz (1985), which show that even low doses of THC impair skills deemed to be important for driving, such as perception, coordination, tracking and vigilance. The present studies also demonstrated that marijuana can have greater effects in laboratory than driving tests. The last study, for example showed a highly significant effect of THC on hand unsteadiness but not on driving in urban traffic.

The studies outlined above indicate that cannabis does cause dose-dependent effects on laboratory based tests of human skills. Furthermore, studies utilising driving simulators and on-road driving also indicate a degree of cannabis induced impairment of driving skills. However in these cases the extent of the impairment indicated from laboratory studies is not replicated in the simulator or in-car studies.

The effects of alcohol on the other hand can be demonstrated both in laboratory studies and in simulated or on-road driving at very much the same dose levels. Explanations for these differences between alcohol and cannabis have been suggested and rest essentially upon the difference in the awareness by the drug taker of the presence of drug impairment.

This in turn may be explained by the present understanding of the quite different ways alcohol and cannabis are known to act on the brain.

Also mentioned above and in other publications our present laws on alcohol and driving have been based upon the scientific principles outlined here and in particular on the results of epidemiological studies. It is pertinent therefore to discuss briefly the nature of the epidemiological studies undertaken to date with cannabis and road crashes.

Epidemiological studies with alcohol are greatly facilitated by the pharmacokinetics of that drug. Alcohol is excreted in the breath and the ratio of the concentration on the breath and in the blood is relatively constant.

Therefore the determination of the concentration of alcohol in the breath (by a 'breathalyser') provides a reasonably and acceptably accurate indication of the blood concentration. It is unfortunate therefore that cannabinoids are not excreted on the breath and the concentration of cannabinoids that can be detected on breath represent only that contained in the 'dead-space air' in the upper respiratory tract.

The cannabinoids so detected do not correlate in any way with the blood concentration. In addition to this the blood concentration of cannabinoids do not show any useful relationship to the degree of impairment or the degree of subjective effects of the drug. The blood concentration of alcohol on the other hand does exhibit a reasonable correlation with the degree of impairment.

These properties of cannabis mean that the determination of the role of cannabis in road crashes by the same techniques of the case-control study as used for alcohol, is not an easy task. The pharmacokinetics of cannabis make this an exceedingly difficult task. The difficulty is not only related to the poor correlation between blood concentration and impairment, but also because it requires the collection of a blood sample-from both the crash case and the controls. The collection of the latter sample is likely to involve a high refusal rate, and this alone would almost certainly invalidate the study. One does not know the reason for the refusal!

The studies that have been undertaken to date can be described within three groups and these are:

(i) Questionnaire based surveys;

(ii) Incidence of drug detection in accident involved drivers; and
(iii) Attempts to assess whether or not the driver who has detectable drug in bloodstream was culpable in the accident.
Studies along the lines outlined above have been reviewed by Simpson.
5.1 Questionnaire based surveys

Questionnaire based surveys by definition depend upon self report data and their reliability is questionable. Furthermore, the incidence of cannabis use and the likelihood of a driver admitting to such use is likely to change across time.

5.2 Incidence of drug detection in crash involved drivers
This technique involves the analysis of blood or urine samples taken from crash involved drivers. The detection of cannabinoids in urine provides information only that the drug has been consumed within the last day or even month. It provides no indication at all of impairment. Therefore only the analysis of a blood sample is likely to be helpful. However, the detection of cannabis in a blood sample does not itself prove impairment or crash culpability. This fact has been well expressed by Compton as follows:

Knowing only the frequency with which crash-involved drivers use drugs does not allow one to know the danger posed by the drugs. It may simply reflect the general drug usage pattern in the driving public at large. For example, finding that 30% of crash-involved drivers have nicotine in their blood does not imply that nicotine was involved in the occurrence of their crashes. It may be that 30% of the general driving population smokes cigarettes and the smoking of cigarettes is unrelated to crash occurrence. Finding that a drug was overrepresented in crash-involved drivers (as compared to non-crash involved drivers) would strongly suggest it played a role in increasing crash risk. However, this approach requires knowing the drug usage rate of the general driving public, something we do not know and can not easily determine.

Furthermore, any comparisons of the incidence of cannabis detections in crash-involved drivers with those of non-crash involved drivers should be collected from a comparable population and at the same time. The patterns of cannabis use vary not only across time but also across populations.
Therefore studies reporting the incidence of drugs in the blood of crash-involved drivers is essentially meaningless without some control of the incidence of drug use in non-crash involved drivers. Nevertheless, such studies have been reported and are reviewed by Simpson who summarised that:
  • Marijuana users are certainly among drivers who are injured in road crashes (suggested by the presence of cannabinoids in urine);
  • More importantly, recent use, as indexed by the presence of THC in blood, is evident in perhaps less than 10% of injured drivers; and
  • When cannabis is detected, there is an 80% chance that alcohol will also be found.

5.3 Attempts to assess whether or not the driver who has detectable drugs in the bloodstream was culpable in the accident

Of the first attempts to assess culpability has been an ongoing series of data collected by McBay of fatal, single vehicle crashes. Culpability in single vehicle crashes is assumed to be that of the driver (assuming no mechanical fault can be found) and the choice of fatal crashes assumes that death occurred shortly after the accident; meaning that drug metabolism ceased at death and therefore the blood sample from the dead body will represent the blood picture at the time of the crash. Cannabis was detected in 7.8% of 600 such cases, but 88% of these also contained alcohol in concentrations which of themselves could have accounted for the crash.


In the absence of a separate control group (as used in the assessment of crash probability with alcohol as described above) an alternative of a 'culpability index' is currently being employed in drug studies. The basic construct is first to formulate a means of determining the responsibility or culpability of a driver involved in a crash.

There have been several means of constructing this 'culpability index' and this must be done with each of the accident cases by observers who have no information as to the drug status of each driver. The responsibility (or culpability) ratio is then determined as the proportion of drug-bearing drivers who were determined to be culpable, to the non-drug bearing drivers who were deemed to be culpable. The null hypothesis predicts a culpability ratio of 1.00 (ie, the drug has had no causal relationship with crashes).

To date there have been six studies employing this technique (two of which have involved the re-analysis of earlier generated data). These are briefly outlined below:

1. Warren and others re-analysed the data of Cimbura and found a culpability index for cannabis of 1.7, the same as that found for alcohol. However, the original data comprised a total of 484 drivers and pedestrians, 3.7% of whom were positive for cannabis. However, 88% of these people were also positive for alcohol. This left a very small number from which to assess a culpability ratio for cannabis alone.

2. Terhune also has previously collected data independently re-analysed to estimate a culpability ratio. All BACs over 0.10% were judged significantly more culpable than the drug-free group. The cannabis group also had a higher culpability ratio than the drug-free group, but this was only marginally significant (58.8% vs 34.4%). This estimation was also compromised by the small sample size for cannabis only (n=17). The cannabis plus alcohol group was analysed separately.

3. Donelson began a very ambitious project but was unfortunately thwarted by funding problems which precluded the complete analysis of the collected data. However, a random sample of 415 cases was analysed. The results cautiously suggested a finding consistent with those of Warren et al. and Terhune above.

4. Williams et al. in a study involving 440 cases, demonstrated as in the above studies that alcohol had a higher culpability ratio compared with culpable drug-free drivers (92% vs 71%). However, those drivers in whom only cannabis was detected were less likely to be responsible for the crashes (53% vs 71%).

5. Terhune et al. reported a very comprehensive study involving 1 882 cases. They found that alcohol was the dominant drug in fatal crashes, although the basic focus of their research was to describe the effect of drugs other than alcohol. They reported that fully 40% of the drivers had only alcohol in their systems and another 11% had alcohol combined with drugs. Among the drivers with BACs at or above 0.10% (n=625) their responsibility rate:

... was an extraordinary 94%, well above that found for any other single substance.

 Of cannabis, the authors stated that while cannabinoids were detected in 7% of the drivers, the psychoactive agent THC was found in only 4%. Of the drivers with only one substance in their system, only 1.1% had cannabis alone, either as the THC the psychoactive compound or had the inactive metabolite carboxy THC. The presence of the inactive metabolite and the absence of detectable THC infers less recent ingestion of cannabisÑassuming an efficient analysis.

 The THC only drivers had a responsibility rate below that of the drug-free driversÑie. as with the study by Williams et al. (1985) they were considered to be less likely to have been a cause of the crash than the drug-free drivers.

 The report also indicated the range of THC concentrations found in the blood. There were 109 cases of THC alone; of these, 22.9% contained what the authors called a 'trace' ie. 1 to 2 nanograms THC per millilitre of blood (ng/ml); 69.7% contained 'low' concentrations between 3 to 19 ng/ml; and 7.3% contained a 'high' concentration of equal to or greater than 20 ng/ml.

6. Drummer reported a study of 1 045 fatalities in New South Wales, Victoria and Western Australia and used the technique of responsibility analysis (culpability index).

 As with other studies, the dominant drug was alcohol, being found overall in 36% of all driver fatalities, 33% of which were over the legal limit of 0.05g%. Cannabis was found in 11% of cases of which 56% (n= 63) also contained alcohol (mean BAC 0.16 g% ± 0.08g%). There was no significant difference in the BAC of the alcohol only drivers and those with alcohol plus cannabis.

 Assessment of the culpability ratio by Drummer provided the same result as those of Williams et al. and Terhune et al; there was a trend to a decrease in relative risk when either THC or the metabolite carboxy THC was measured in blood or urine. The relative risk was 0.6 relative to drug-free drivers, although this was not significant statistically.


The relative risk for drivers with alcohol plus cannabis was also greater than that for the control group, but this culpability ratio was no different from the alcohol only group. Also in this study (as indicated above), there was no significant difference in the BAC of the alcohol-only drivers and those with alcohol plus cannabis.

The same finding was reported by Terhune who also suggested that the high levels of alcohol are primarily responsible for the increased crash risk.

Therefore the effects of alcohol in road crashes are really profound. The studies reviewed here using the method of 'responsibility analysis' have confirmed the information already established by the case-control methods-that alcohol is the dominant drug associated with risky and dangerous driving and road crashes.
There have been suggestions throughout the studies reviewed here that the crash responsibility rates associated with the low BAC plus other drug, might be higher than in the low alcohol-only groups. The interaction of other drugs and alcohol (including cannabis) require further study using epidemiological techniques. One must remember the description by Perez-Reyes of the effect of the order of administration of alcohol and cannabis in these interaction studies.

The most recent of the reports of studies of the effects of cannabis on actual driving performance included a summary of the published literature on marijuana and driving. They concluded this review with the following paragraph:

The foremost impression one gains from reviewing the literature is that no clear relationship has ever been demonstrated between marijuana smoking and either seriously impaired driving performance or the risk of accident involvement.

The epidemiological evidence, as limited as it is, shows that the combination of THC and alcohol is over-represented in injured and dead drivers and more so in those who actually caused the accidents to occur. Yet there is little if any evidence to indicate that drivers who have used marijuana alone are any more likely to cause serious accidents than drug free drivers. To a large extent, the results from driving simulator and closed-course tests corroborate the epidemiological findings by indicating that THC in single inhaled doses up to 250 µg/kg has relatively minor effects on driving performance, certainly less than BACs in the range of 0.08Ê-Ê0.10g%.

Apart from the above, a very important finding in the reviewed studies is the difference in the drug users' awareness of the effect of the drugs alcohol and cannabis. Alcohol use is accompanied by increased confidence, an impairment of judgement to the extent that driving behaviour becomes more risky, with faster speeds and a greater willingness to take risks. Cannabis use on the other hand, is accompanied by compensatory driving behaviour, including a reduced willingness to take risks and slower driving speeds. Indeed the compensation was described by Robbe and O'Hanlon in the following manner:

Very importantly our city driving study showed that drivers who drank alcohol overestimated their performance quality whereas those who smoked marijuana underestimated it. Perhaps as a consequence, the former invested no special effort for accomplishing the task whereas the latter did, and successfully. This evidence strongly suggests that alcohol encourages risky driving whereas THC encourages greater caution, at least in experiments.

The task of driving has been described as a 'self-paced' task. That is, drivers choose their own levels of task difficulty. There is a difference therefore between a driver's skills performance, as measured in individual laboratory tasks and driver behaviour. Driver performance, or skills performance is what a driver can do. Driver behaviour is what a driver actually does.

Driving skills (or driver skills performance) differ very widely within a community. Some of us may be extremely cautious and others much less so. The correlation between driver skills and crash probability is not as great as many may imagine. For example, it is held by many that superior driver skills lead to reduced crashes and this led to the concept of 'advanced driver training'. Indeed, an editor of a road magazine claimed: 'I have for many years claimed that the licensed racer is far safer than ordinary chaps, on the grounds of practised skills, mental ability, cognisance of hazards in driving, keen interest in driving as well, and so on.'

In order to examine the possibility that unusually skilled drivers really did have different on-the-road driving records from the average driver, a comparison was made of the on-the-road driving records of a group of licensed racing drivers with those of other drivers matched for such characteristics as sex and age, etc. What they found was that in all measures of traffic violations including crashes, speeding violations, other moving violations as well as non-moving violations, the rates for the racing drivers exceed those of the comparison drivers, in most cases by a considerable margin.

In the light of the above, Terhune et al. asked the following questions:
A nagging question which qualifies conclusions from epidemiological studies of drugs in crashes is: If certain drugs are linked to elevated crash risks, how much of the elevation is due to characteristics of the people who use these drugs?

For example, Terhune in a literature review remarked that research revealed a striking similarity between the personal correlates of marijuana use and the correlates of crash involvement. Rebellious, deviant, youthful males were prominent among marijuana users and among those in crashes. Jessor et al. also addresses these issues.

A general conclusion made by Robbe and O'Hanlon when discussing the results of their study and of their review of the literature is worth citing here as a general conclusion to this review:
In summary, this program of research has shown that marijuana, when taken alone, produces a moderate degree of driving impairment which is related to the consumed THC dose.
The impairment manifests itself mainly in the ability to maintain a steady lateral position on the road, but its magnitude is not exceptional in comparison with changes produced by many medicinal drugs and alcohol.

Drivers under the influence of marijuana retain insight in their performance and will compensate where they can, for example, by slowing down or increasing effort. As a consequence THC's adverse effects on driving performance appear relatively small. Still we can easily imagine situations where the influence of marijuana smoking might have an exceedingly dangerous effect ie, emergency situations which put high demands on the driverÕs information processing capacity, prolonged monotonous driving, and after THC has been taken with other drugs especially alcohol.

We therefore agree with Moskowitz's conclusion that 'any situation in which safety both for self and others depends on alertness and capability of control of man-machine interaction precludes the use of marijuana'.
However, the magnitude of marijuana's relative to many other drugs' effects also justify Geringer's (1988) conclusion that 'marijuana impairment presents a real, but secondary, safety risk; and that alcohol is the leading drug-related risk factor'. Of the many psychotropic drugs, licit and illicit, that are available and used by people who subsequently drive, marijuana may well be among the least harmful.

Campaigns to discourage the use of marijuana by drivers are certainly warranted. But concentrating a campaign on marijuana alone may not be in proportion to the safety problem it causes.



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Marijuana And Actual Driving Performance

Conducted on behalf of: U.S. Department of Transportation,
National Highway Traffic Safety Administration
(DOT HS 808 078), Final Report, November 1993

Conducted by: HWJ Robbe Institute for Human Psychopharmacology,
University of Maastricht, P.O. Box 616, NL-6200 MD,<
Maastricht, The Netherlands



Marijuana's effects on actual driving performance were assessed in a series of three studies wherein dose-effect relationships were measured in actual driving situations that progressively approached reality. The first was conducted on a highway closed to other traffic. Subjects (24) were treated on separate occasions with THC 100, 200 and 300 g/kg, and placebo. They performed a 22-km road tracking test beginning 30 and 90 minutes after smoking.

Their lateral position variability increased significantly after each THC dose relative to placebo in a dose-dependent manner for two hours after smoking. The second study was conducted on a highway in the presence of other traffic. Subjects (16) were treated with the same THC doses as before. They performed a 64-km road tracking test preceded and followed by 16-km car following tests.

Results confirmed those of the previous study. Car following performance was only slightly impaired. The third study was conducted in high-density urban traffic. Separate groups of 16 subjects were treated with 100 g/kg THC and placebo; and, ethanol (mean BAC .034 g%) and placebo. Alcohol impaired performance relative to placebo but subjects did not perceive it. THC did not impair driving performance yet the subjects thought it had. These studies show that THC in single inhaled doses up to 300 g/kg has significant, yet not dramatic, dose-related impairing effects on driving performance.


This article describes the results of a research program that was set up to determine the dose-response relationship between marijuana and objectively and subjectively measured aspects of real world driving; and to determine whether it is possible to correlate driving performance impairment with plasma concentrations of the drug or a metabolite. The program consisted of three driving studies in which a variety of driving tasks were employed, including: maintenance of a constant speed and lateral position during uninterrupted highway travel, following a leading car with varying speed on a highway, and city driving. A laboratory study preceded the driving studies for identifying the highest THC dose to be administered in the subsequent studies.


Subjects in all studies were recreational users of marijuana or hashish, i.e., smoking the drug more than once a month, but not daily. They were all healthy, between 21 and 40 years of age, had normal weight and binocular acuity, and were licensed to drive an automobile. Furthermore, law enforcement authorities were contacted, with the volunteers' consent, to verify that they had no previous arrests or convictions for drunken driving or drug trafficking.

Each subject was required to submit a urine sample immediately upon arrival at the test site. Samples were assayed qualitatively for the following common 'street drugs' (or metabolites): cannabinoids, benzodiazepines, opiates, cocaine, amphetamines and barbiturates. In addition a breath sample was analyzed for the presence of alcohol. Blood samples were repeatedly taken after smoking by venepuncture. Quantitative analysis of THC and THC-COOH in plasma was performed by gas chromatography/mass spectrometry (gc/ms) using deuterated cannabinoids as internal standards.

Marijuana and placebo marijuana cigarettes were supplied by the U.S. National Institute on Drug Abuse. The lowest and highest THC concentrations in the marijuana cigarettes used in the studies were 1.75% and 3.57%, respectively. Subjects smoked the administered cigarettes through a plastic holder in their customary fashion.

Subjects were accompanied during every driving test by a licensed driving instructor. A redundant control system in the test vehicle was available for controlling the car, should emergency situations arise.

In each study, subjects repeatedly performed certain simple laboratory tests (e.g. critical instability tracking, hand and posture stability), estimated their levels of intoxication and indicated their willingness to drive under several specified conditions of urgency. In addition, heart rate and blood pressure were measured. Results of these measurements are reported elsewhere (Robbe, 1994).



Twenty-four subjects, equally comprised of men and women, participated in this study. They were allowed to smoke part or all of the THC content in three cigarettes until achieving the desired psychological effect. The only requirement was to smoke for a period not exceeding 15 minutes. When subjects voluntarily stopped smoking, cigarettes were carefully extinguished and retained for subsequent gravimetric estimation of the amount of THC consumed.


Six subjects consumed one cigarette, thirteen smoked two and four smoked three (data from one male subject were excluded from the results because no drug was found in his plasma after smoking). The average amount of THC consumed was 20.8 mg, after adjustment for body weight, 308 g/kg. It should be noted that these amounts of THC represent both the inhaled dose and the portion that was lost through pyrolysis and side- stream smoke during the smoking process. There were no significant differences between males and females, nor between frequent and infrequent users, with respect to the weight adjusted preferred dose. It was decided that the maximum dose for subsequent driving studies would be 300 g/kg.



The first driving study was conducted on a highway closed to other traffic. The same twelve men and twelve women who participated in the laboratory study served again as the subjects.

They were treated on separate occasions with marijuana cigarettes containing THC doses of 0 (placebo), 100, 200, and 300 g/kg. Treatments were administered double- blind and in a counterbalanced order. On each occasion, subjects performed a road-tracking test beginning 40 minutes after initiation of smoking and repeated one hour later. The test involved maintaining a constant speed at 90 km/h and a steady lateral position between the delineated boundaries of the traffic lane. Subjects drove 22 km on a primary highway and were accompanied by a licensed driving instructor.

The primary dependent variable was the standard deviation of lateral position (sdlp), which has been shown to be both highly reliable and very sensitive to the influence of sedative medicinal drugs and alcohol. Other dependent variables were mean speed, and standard deviations of speed and steering wheel angle. Blood samples were taken 10 minutes before the driving tests (i.e. 30 and 90 minutes after initiation of smoking, respectively).


All subjects were willing and able to finish the driving tests without great difficulty. Data from one male subject were excluded from the results because no drug was found in his plasma after smoking.

Figure 1 demonstrates that marijuana impairs driving performance as measured by an increase in lateral position variability: all three THC doses significantly affected sdlp relative to placebo (p<.012, .001 & .001, for the 100, 200 & 300 g/kg conditions, respectively.

The Dose by Time effect was not significant indicating that impairment after marijuana was the same in both trials. Marijuana's effects on sdlp were compared to those of alcohol obtained in a very similar study by Louwerens et al. (1987). It appeared that the effects of the various administered THC doses (100-300 g/kg) on sdlp were equivalent to those associated with bacs in the range of 0.03-0.07 g%.

Other driving performance measures were not significantly affected by THC. Plasma concentrations of the drug were clearly related to the administered dose and time of blood sampling but unrelated to driving performance impairment.



The second driving study was conducted on a highway in the presence of other traffic and involved both a road-tracking and a car-following test. A new group of sixteen subjects, equally comprised of men and women, participated in this study. A conservative approach was chosen in designing the present study in order to satisfy the strictest safety requirements.

That is, the study was conducted according to an ascending dose series design where both active drug and placebo conditions were administered, double- blind, at each of three THC dose levels. THC doses were the same as those used in the previous study, namely 100, 200, and 300 g/kg. Cigarettes appeared identical at each level of treatment conditions. If any subject would have reacted in an unacceptable manner to a lower dose, he/she would not have been permitted to receive a higher dose.

The subjects began the car-following test 45 minutes after smoking. The test was performed on a 16 km segment of the highway and lasted about 15 minutes. After the conclusion of this test, subjects performed a 64-km road-tracking test on the same highway which lasted about 50 minutes. At the conclusion of this test, they participated again in the car-following test. Blood samples were taken both before the first and after the last driving test (i.e. 35 and 190 minutes after initiation of smoking, respectively).

The road-tracking test was the same as in the previous study except for its duration and the presence of other traffic. The car-following test involved attempting to match velocity with, and maintain a constant distance from a preceding vehicle as it executed a series of deceleration/acceleration maneuvers.

The preceding vehicle's speed would vary between 80 and 100 km/h and the subject was instructed to maintain a 50 m distance however the preceding vehicle's speed might vary. The duration of one deceleration and acceleration maneuver was approximately 50 seconds and six to eight of these maneuvers were executed during one test, depending upon traffic density. The subject's average reaction time to the movements of the preceding vehicle, mean distance and coefficient of variation of distance during maneuvers were taken as the dependent variables from this.


All subjects were able to complete the series without suffering any untoward reaction while driving. Data from one female subject were excluded from the results because no drug was found in her plasma after smoking.

Road-tracking performance in the standard test was impaired in a dose- related manner by THC and confirmed the results obtained in the previous closed highway study (Figure 2). The 100 g/kg dose produced a slight elevation in mean sdlp, albeit not statistically significant (p<.13). The 200 g/kg dose produced a significant (p<.023) elevation, of dubious practical relevance. The 300 g/kg dose produced a highly significant (p<.007) elevation which may be viewed as practically relevant. After marijuana smoking, subjects drove with an average speed that was only slightly lower than after placebo and very close to the prescribed level.

In the car-following test, subjects maintained a distance of 45-50 m while driving in the successive placebo conditions. They lengthened mean distance by 8, 6 and 2 m in the corresponding THC conditions after 100, 200 and 300 g/kg, respectively. The initially large drug-placebo difference and its subsequent decline is a surprising result. Our explanation for this observation is that the subjects' caution was greatest the first time they undertook the test under the influence of THC and progressively less thereafter. The reaction time of the subjects to changes in the preceding vehicle's speed increased following THC treatment, relative to placebo.

The administered THC dose was inversely related to the change in reaction time, as it was to distance. However, increased reaction times were partly due to longer distance (i.e. the longer the distance to the preceding vehicle, the more difficult it is to perceive changes in its speed). Statistical adjustment for this confounding variable resulted in smaller and non- significant increases in reaction time following marijuana treatment, the greatest impairment (0.32 s) being observed in the first test following the lowest THC dose (Figure 3). Distance variability followed a similar pattern as mean distance and reaction time; the greatest impairment was found following the lowest dose. As in the previous study, plasma concentrations of the drug were not related to driving impairment.



The program proceeded into the third driving study, which involved tests conducted in high-density urban traffic. There were logical and safety reasons for restricting the THC dose to 100 g/kg. It was given to a new group of 16 regular marijuana (or hashish) users, along with a placebo. For comparative purposes, another group of 16 regular users of alcohol, but not marijuana, were treated with a modest dose of their preferred recreational drug, ethanol, and again placebo, before undertaking the same city driving test. Both groups were equally comprised of men and women.

Marijuana was administered to deliver 100 g/kg THC. The driving test commenced 30 minutes after smoking. The alcohol dose was chosen to yield a bac approaching 0.05 g% when the driving test commenced 45 minutes after onset of drinking. Active drug and placebo conditions were administered double-blind and in a counterbalanced order in each group. Blood samples were taken immediately prior to and following all placebo and drug driving tests (i.e. 20 and 80 minutes after initiation of smoking, or 35 and 95 minutes after initiation of drinking).

Driving tests were conducted in daylight over a constant 17.5 km route within the city limits of Maastricht. Subjects drove their placebo and active- drug rides through heavy, medium and low density traffic on the same day of the week, and at the same time of day. Two scoring methods were employed in the present study.

The first, a 'molecular' approach adopted from Jones (1978), involved the employment of a specially trained observer who applied simple and strict criteria for recording when the driver made or failed to make each in a series of observable responses at predetermined points along a chosen route.

The second, a 'molar' approach, required the driving instructor acting as the safety controller during the tests to retrospectively rate the driver's performance using a shortened version of the Royal Dutch Tourist Association's Driving Proficiency Test. In total, 108 items were dichotomously scored, as either pass or fail. Total test performance was measured by the percentage items scored as 'pass'. Subscores were calculated for vehicle checks, vehicle handling, traffic maneuvers, observation and understanding of traffic, and turning'. This method has been applied previously to show the impairing effects of alcohol and diazepam (De Gier, 1979; De Gier et al., 1981).


Data from two male subjects in the marijuana group were excluded from the results because neither THC nor THC-COOH was found in their plasma after smoking.

Neither alcohol nor marijuana significantly affected driving performance measures obtained by the molecular approach, indicating that it may be relatively insensitive to drug-induced changes. The molar approach was more sensitive. Table 1 shows that a modest dose of alcohol (bac=0.034 g%) produced a significant impairment in city driving, relative to placebo. More specifically, alcohol impaired both vehicle handling and traffic maneuvers. Marijuana, administered in a dose of 100 g/kg THC, on the other hand, did not significantly change mean driving performance as measured by this approach.

Subjects' ratings of driving quality and effort to accomplish the task were strikingly different from the driving instructor's ratings. Both groups rated their driving performance following placebo as somewhat better than 'normal'. Following the active drug, ratings were significantly lower (35%, p<.009) in the marijuana, but not (5%, ns) in the alcohol group. Perceived effort to accomplish the driving test was about the same in both groups following placebo. Following the active drug, a significant (p<.033) increase in perceived effort was reported by the marijuana, but not the alcohol group.

Thus, there is evidence that subjects in the marijuana group were not only aware of their intoxicated condition, but were also attempting to compensate for it. These seem to be important findings. They support both the common belief that drivers become overconfident after drinking alcohol and investigators' suspicions that they become more cautious and self- critical after consuming low doses of THC, as smoked marijuana.

Drug plasma concentrations were neither related to absolute driving performance scores nor to the changes that occurred from placebo to drug conditions. With respect to THC, these results confirm the findings in previous studies. They are somewhat surprising for alcohol but may be due to the restricted range of ethanol concentrations in the plasma of different subjects.


The results of the studies corroborate those of previous driving simulator and closed-course tests by indicating that THC in inhaled doses up to 300 g/kg has significant, yet not dramatic, dose-related impairing effects on driving performance (cf. Smiley, 1986). Standard deviation of lateral position in the road-tracking test was the most sensitive measure for revealing THC's adverse effects.

This is because road-tracking is primarily controlled by an automatic information processing system which operates outside of conscious control. The process is relatively impervious to environmental changes but highly vulnerable to internal factors that retard the flow of information through the system. THC and many other drugs are among these factors.

When they interfere with the process that restricts road-tracking error, there is little the afflicted individual can do by way of compensation to restore the situation. Car-following and, to a greater extent, city driving performance depend more on controlled information processing and are therefore more accessible for compensatory mechanisms that reduce the decrements or abolish them entirely.

THC's effects on road-tracking after doses up to 300 g/kg never exceeded alcohol's at bacs of 0.08 g%; and, were in no way unusual compared to many medicinal drugs' (Robbe, 1994; Robbe and O'Hanlon, 1995; O'Hanlon et al., 1995). Yet, THC's effects differ qualitatively from many other drugs, especially alcohol. Evidence from the present and previous studies strongly suggests that alcohol encourages risky driving whereas THC encourages greater caution, at least in experiments. Another way THC seems to differ qualitatively from many other drugs is that the former's users seem better able to compensate for its adverse effects while driving under the influence.

Inter-subject correlations between plasma concentrations of the drug and driving performance after every dose were essentially nil, partly due to the peculiar kinetics of THC. It enters the brain relatively rapidly, although with a perceptible delay relative to plasma concentrations.

Once there, it remains even at a time when plasma concentrations approach or reach zero. As a result, performance may still be impaired at the time that plasma concentrations of the drug are near the detection limit. This is exactly what happened in the first driving study. Therefore an important practical implications of the study is that is not possible to conclude anything about a driver's impairment on the basis of his/her plasma concentrations of THC and THC-COOH determined in a single sample.

Although THC's adverse effects on driving performance appeared relatively small in the tests employed in this program, one can still easily imagine situations where the influence of marijuana smoking might have a dangerous effect; i.e., emergency situations which put high demands on the driver's information processing capacity, prolonged monotonous driving, and after THC has been taken with other drugs, especially alcohol. Because these possibilities are real, the results of the present studies should not be considered as the final word. They should, however, serve as the point of departure for subsequent studies that will ultimately complete the picture of THC's effects on driving performance.


De Gier JJ (1979) A subjective measurement of the influence of ethyl/alcohol in moderate doses on real driving performances.Blutalkohol, 16, 363-370.

De Gier JJ, 't Hart BJ, Nelemans FA and Bergman H (1981) Psychomotor performance and real driving performance of outpatients receiving diazepam. Psychopharmacology, 73, 340-347.

Jones MH (1978) Driver Performance Measures for the Safe Performance Curriculum. Traffic Safety Center, Institute of Safety and Systems Management, University of South California, Los Angeles, CA (DOT HS 803 461).

Louwerens JW, Gloerich ABM, de Vries G, Brookhuis KA and O'Hanlon JF (1987). The relationship between drivers' blood alcohol concentration (bac) and actual driving performance during high speed travel. Pages 183-192 in PC Noordzij and R Roszbach, eds., Alcohol, Drugs and Traffic Safety. Proceedings of the 10th International Conference on Alcohol, Drugs and Traffic Safety. Excerpta Medica, Amsterdam.

O'Hanlon JF, Vermeeren A, Uiterwijk MMC, van Veggel LMA and Swijgman HF (1995) Anxiolytics' effects on the actual driving performance of patients and healthy volunteers in a standardized test: an integration of three studies. Neuropsychobiology, 31:81-88.

Robbe HWJ (1994). Influence of Marijuana on Driving. PhD thesis, Institute for Human Psychopharmacology, University of Limburg, Maastricht.

Robbe HWJ and O'Hanlon JF (1995) Acute and subchronic effects of paroxetine and amitriptyline on actual driving, psychomotor performance and subjective assessments in healthy volunteers. European Neuropsychopharmacology, 5:35-42.

Smiley AM (1986). Marijuana: On-road and driving simulator studies. Alcohol, Drugs and Driving: Abstracts and Reviews 2: 121-134.


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Cannabis and Driving: A Scientific and Rational Review

By Paul Armentano

Policy debates regarding marijuana law reform invariably raise the question: "How does society address concerns regarding pot use and driving?" The subject is worthy of serious discussion. NORML’s Board of Directors addressed this issue by ratifying a “no driving” clause to the organization’s “Principles of Responsible Cannabis Use” stating, “Although cannabis is said by most experts to be safer with motorists than alcohol and many prescription drugs, responsible cannabis consumers never operate motor vehicles in an impaired condition.”

Nevertheless, questions remain regarding the degree to which smoking cannabis impairs actual driving performance. Unlike alcohol, which is known to increase drivers’ risk-taking behavior and is a primary contributor in on-road accidents, marijuana’s impact on psychomotor skills is subtle and its real-world impact in automobile crashes is conflicting.

Drugged Driving: True Threat Or False Panic?

Survey data indicates that approximately 112 million Americans (46 percent of the US population) have experimented with the use of illicit substances. Of these, more than 20 million (8.3 percent of the population) self-identify as “current” or “monthly” users of illicit drugs, and more than 10 million Americans say that they’ve operated a motor vehicle while under the influence of an illicit substance in the past year. These totals, while far from negligible, suggest that the prevalence of illicit drug use among US drivers is far less than the prevalence of alcohol among this same population.

To date, “[The] role of drugs as a causal factor in traffic crashes involving drug-positive drivers is still not well understood.” While some studies have indicated that illicit drug use is associated with an increased risk of accident, a relationship has not been established regarding the use of psychoactive substances and crash severity.

Drivers under the influence of illicit drugs do experience an enhanced fatality risk compared to sober drivers. However, this risk is approximately three times lower than the fatality risk associated with drivers who operate a vehicle above or near the legal limit for alcohol intoxication. According to one recent review: “The risk of all drug-positive drivers compared to drug-free drivers is similar to drivers with a blood alcohol concentration of 0.05%. The risk is also similar to drivers above age 60 compared to younger drivers [around age 35].”

Marijuana is the most common illicit substance consumed by motorists who report driving after drug use. Epidemiological research also indicates that cannabis is the most prevalent illicit drug detected in fatally injured drivers and motor vehicle crash victims. Reasons for this are twofold. One, pot is by far the most widely used illicit drug among the US population, with nearly one out of two Americans admitting having tried it.Two, marijuana is the most readily detectable illicit drug in toxicological tests.

Marijuana’s primary psychoactive compound, THC, may be detected in blood for several hours, and in some extreme cases days after past use, long after any impairing effects have worn off. In addition, non-psychoactive byproducts of cannabis, known as metabolites, may be detected in the urine of regular users for days or weeks after past use. (Other common drugs of abuse, such as cocaine or methamphetamine, do not possess such long half-lives.)

Therefore, pot’s prevalence in toxicological evaluations of US drivers does not necessarily indicate that it is a frequent or significant causal factor in auto accidents. Rather, its prevalence affirms that cannabis remains far more popular and is far more easily detectable on drug screening tests than other controlled substances.

Cruising On Cannabis: Clarifying The Debate

While it is well established that alcohol consumption increases accident risk, evidence of marijuana’s culpability in on-road driving accidents and injury is far less clear. Although acute cannabis intoxication following smoking has been shown to mildly impair psychomotor skills, this impairment is seldom severe or long lasting.

In closed course and driving simulator studies, marijuana’s acute effects on psychomotor performance include minor impairments in tracking (eye movement control) and reaction time, as well as variation in lateral positioning, headway (drivers under the influence of cannabis tend to follow less closely to the vehicle in front of them), and speed (drivers tend to decrease speed following cannabis inhalation).

In general, these variations in driving behavior are noticeably less consistent or pronounced than the impairments exhibited by subjects under the influence of alcohol. Also, unlike subjects impaired by alcohol, individuals under the influence of cannabis tend to be aware of their impairment and try to compensate for it accordingly, either by driving more cautiously or by expressing an unwillingness to drive altogether.

As a result, cannabis-induced variations in performance do not appear to play a significant role in on-road traffic accidents when THC levels in a driver's blood are low and/or cannabis is not consumed in combination with alcohol. For example, a 1992 National Highway Traffic Safety Administration review of the role of drug use in fatal accidents reported, “There was no indication that cannabis itself was a cause of fatal crashes” among drivers who tested positive for the presence of the drug.

A more recent assessment by Blows and colleagues noted that self-reported recent use of cannabis (within three hours of driving) was not significantly associated with car crash injury after investigators controlled for specific cofounders (e.g., seat-belt use, sleepiness, etc.)

A 2004 observational case control study published in the journal Accident, Analysis and Prevention reported that only drivers under the influence of alcohol or benzodiazepines experience an increased crash risk compared to drug-free controls. Investigators did observe increased risks – though they were not statistically significant – among drivers using amphetamines, cocaine and opiates, but found, “No increased risk for road trauma was found for drivers exposed to cannabis.”

A handful of more recent studies have noted a positive association between very recent cannabis exposure and a gradually increased risk of vehicle accident. Typically, these studies reveal that drivers who possess THC/blood concentrations above 5ng/ml – implying cannabis inhalation within the past 1-3 hours – experience an elevated risk of accident compared to drug-free controls. (Motorists who test positive for the presence of THC in the blood at concentrations below this threshold typically do not have an increased risk compared to controls.) However, this elevated risk is below the risk presented by drivers who have consumed even small quantities of alcohol.

Two recent case-controlled studies have assessed this risk in detail. A 2007 case-control study published in the Canadian Journal of Public Health reviewed 10-years of US auto-fatality data. Investigators found that US drivers with blood alcohol levels of 0.05% – a level well below the legal limit for intoxication – were three times as likely to have engaged in unsafe driving activities prior to a fatal crash as compared to individuals who tested positive for marijuana.

A 2005 review of auto accident fatality data from France showed similar results, finding that drivers who tested positive for any amount of alcohol had a four times greater risk of having a fatal accident than did drivers who tested positive for marijuana in their blood. In the latter study, even drivers with low levels of alcohol present in their blood (below 0.05%) experienced a greater elevated risk as compared to drivers who tested positive for high concentrations of cannabis (above 5ng/ml). Both studies noted that overall few traffic accidents appeared to be attributed to driver’s operating a vehicle while impaired by cannabis.

Defining A Rational ‘Drugged Driving’ Policy

The above review illustrates the need for further education and understanding regarding the effects of cannabis upon driving behavior. While pot’s adverse impact on psychomotor skills is less severe than the effects of alcohol, driving under the acute influence of cannabis still may pose an elevated risk of accident in certain situations. However, because marijuana’s psychomotor impairment is subtle and short-lived, consumers can greatly reduce this risk by refraining from driving for a period of several hours following their cannabis use.

By contrast, motorists should never be encouraged to operate a vehicle while smoking cannabis. Drivers should also be advised that engaging in the simultaneous use of both cannabis and alcohol can significantly increase their risk of accident compared to the consumption of either substance alone. Past use of cannabis, as defined by the detection of inactive cannabis metabolites in the urine of drivers, is not associated with an increased accident risk.

Educational or public service campaigns targeting drugged driving behavior should particularly be aimed toward the younger driving population age 16 to 25 – as this group is most likely use cannabis and report having operated a motor vehicle shortly after consuming pot. In addition, this population may have less driving experience, may be more prone to engage in risk-taking behavior, and may be more naïve to pot’s psychoactive effects than older, more experienced populations.

This population also reports a greater likelihood for having driven after using cannabis in combinations with other illicit drugs or alcohol. Such an educational campaign was recently launched nationwide in Canada by the Canadian Public Health Association and could readily be replicated in the United States. Arguably, such a campaign would enjoy enhanced credibility if coordinated by a private public health association or traffic safety organization, such as the American Public Health Association or the AAA Automobile Club, as opposed to the federal Office of National Drug Control Policy – whose previous public service campaigns have demonstrated limited influence among younger audiences.

Finally, increased efforts should be made within the law enforcement community to train officers and DREs (drug recognition experts) to better identify drivers who may be operating a vehicle while impaired by marijuana.

In Australia, efforts have been made to adapt elements of the roadside Standardized Field Sobriety Test to make it sensitive to drivers who may be under the influence of cannabis. Scientific evaluations of these tests have shown that subjects’ performance on the modified SFSTs may be positively associated with dose-related levels of marijuana impairment. Similarly, clinical testing for cannabis impairment among suspected drugged drivers in Norway has been positively associated with identifying drivers with THC/blood concentrations above 3ng/ml.

Though the development of such cannabis-specific impairment testing is still in its infancy, an argument may be made for the provisional use of such tests by specially trained members of law enforcement. In addition, the development of cannabis-sensitive technology to rapidly identify the presence of THC in drivers, such as a roadside saliva test, would provide utility to law enforcement in their efforts to better identify intoxicated drivers.

 The development of such technology would also increase public support for the taxation and regulation of cannabis by helping to assuage concerns that liberalizing marijuana policies could potentially lead to an increase in incidences of drugged driving. Such concerns are a significant impediment to the enactment of marijuana law reform, and must be sufficiently addressed before a majority of the public will embrace any public policy that proposes regulating adult cannabis use like alcohol.

Paul Armentano is the Deputy Director of NORML and the NORML Foundation. Mr. Armentano is an expert in the field of marijuana policy, health, and pharmacology. He has spoken at numerous national conferences and legal seminars, testified before several state legislatures and federal bodies, and assisted dozens of criminal defense attorneys in cases pertaining to the use of medicinal cannabis and drugged driving. He has attended various international conferences on the subject of cannabis and psychomotor impairment, including those sponsored by the Society of Forensic Toxicologists (SOFT) and the The International Council on Alcohol, Drugs & Traffic Safety (ICADTS), and coordinated lobbying efforts to successfully liberalize so-called ‘zero tolerant’ drugged driving laws in Virginia and Ohio. He is the author of the 2006 cover story, "Cannabis and Zero Tolerance Per Se DUID Legislation: A Special (and Problematic) Case," for Florida Defender, the journal of the Florida Association of Criminal Defense Lawyers. (FACDL). He may be contacted via e-mail at:


 Adopted by NORML’s Board of Directors, February 3, 1996. Read all of NORML’s “Principles of Responsible Use

US Department of Justice, Bureau of Justice Statistics. Drug and Crime Facts: Drug Use Among the General Population. Online document accessed November 24, 2007.

US Department of Health and Human Services, Substance and Mental Health Services Association, Office of Applied Studies. 2006 National Survey on Drug Use and Health: National Results. Online document accessed November 24, 2007.

 US Department of Transportation, National Highway Traffic Safety Administration. State of Knowledge of Drugged Driving: FINAL REPORT. September 2003.

Smink et al. 2005. Drug use and the severity of traffic accident. Accident, Analysis and Prevention 37: 427-433.

Franjo Grotenhermen. Drugs and Driving: Review for the National Treatment Agency, UK. Nova-Institut (Germany). November 2007.

US Department of Health and Human Services, Substance and Mental Health Services Association, Office of Applied Studies. Driving After Drug or Alcohol Use, 1998. Online document accessed November 24, 2007.

US Department of Transportation. 2003. op. cit.

October 23-24, 2002 CNN/Time poll conducted by Harris Interactive.

Skopp et al. 2003. Serum cannabinoid levels 24 to 48 hours after cannabis smoking. Archives of Criminology (Germany) 212: 83-95.

Paul Cary. 2005. The marijuana detection window: Determining the length of time cannabinoids will remain detectable in urine following smoking. Drug Court Review 5: 23-58.

According to the US Department of Transportation, 2003. op. cit., “Experimental research on the effects of cannabis … indicat[e] that any effects … dissipate quickly after one hour.”

Grotenhermen. 2007. op. cit. and US Department of Transportation. 2003. op. cit. Other summaries include: Ramaekers et al. 2006. Cognition and motor control as a function of Delta-9-THC concentration in serum and oral fluid: Limits of impairment. Drug and Alcohol Dependence 85: 114-122; David Hadorn. “A Review of Cannabis and Driving Skills,” In: The Medicinal Uses of Cannabis and Cannabinoids. (eds: Guy et al). Pharmaceutical Press, 2004; Canadian Senate Special Committee on Illegal Drugs, Cannabis: Summary Report: Our Position for a Canadian Public Policy. 2002. (See specifically: Chapter 8: “Driving Under the Influence of Cannabis”); Alison Smiley. “Marijuana: On-Road and Driving-Simulator Studies,” In: The Health Effects of Cannabis. (eds. Kalant et al) Canadian Centre for Addiction and Mental Health, 1999.

David Hadorn. 2004. op. cit. and US Department of Transportation. 2003. op. cit.

According to the US Department of Transportation, 2003. op. cit., “The extensive studies by Robbe and O’Hanlon (1993), revealed that under the influence of marijuana, drivers are aware of their impairment, and when the experimental task allows it, they tend to actually decrease speed, avoid passing other cars, and reduce other risk-taking behaviors.”

Menetrey et al. 2005. Assessment of driving capability through the use of clinical and psychomotor tests in relation to blood cannabinoid levels following oral administration of 20mg dronabinol or of a cannabis decoction made with 20 and 60mg delta-9-THC. Journal of Analytical Toxicology 29: 327-338.

United Kingdom Department of Environment, Transport and the Regions, Road Safety Division Cannabis and Driving: A Review of the Literature and Commentary. Online document accessed November 24, 2007. “Overall, we conclude that the weight of the evidence indicates that … there is no evidence that consumption of cannabis alone increases the risk of culpability for traffic crash fatalities or injuries for which hospitalization occurs, and may reduce those risks.”

Gregory Chesher and Marie Longo. “Cannabis and Alcohol in Motor Vehicle Accidents,” In: Cannabis and Cannabinoids: Pharmacology, Toxicology, and Therapeutic Potential. (eds. Grotenhermen et al.) Haworth Press, 2002.

US Department of Transportation, National Highway Traffic Safety Administration. The Incidence and Role of Drugs in Fatally Injured Drivers: Final Report. October 1992.

Blows et al. 2004. Marijuana use and car crash injury. Addiction 100: 605-611.

Movig et al. 2004. Psychoactive substance use and the risk of motor vehicle accidents. Accident Analysis and Prevention 36: 631-636.

Huestis et al. 1992. Blood cannabinoids: Absorption of THC and formation of 11-OH-THC and THCCOOH during and after smoking marijuana. Journal of Analytical Toxicology 16: 276-282.

Mushoff et al. 2006. Review of biologic matrices (urine, blood, hair) as indicators of recent or ongoing cannabis use. Therapeutic Drug Monitor 2: 155-163.

Drummer et al. 2004. The involvement of drugs in drivers killed in Australian road traffic crashes. Accident, Analysis and Prevention 36: 239-248.

Grotenhermen et al. 2007. Developing per se limits for driving under cannabis. Addiction (E-pub ahead of print).

Grotenhermen. 2007. op. cit.

Bedard et al. 2007. The impact of cannabis on driving. Canadian Journal of Public Health 98: 6-11.

Laumon et al. 2005. Cannabis intoxication and fatal road crashes in France: a population base case-control study. British Medical Journal 331: 1371-1377.

Ramaekers et al. 2004. Dose related risk of motor vehicle crashes after cannabis use. Drug and Alcohol Dependence 73: 109-119. “Experimental studies have shown alcohol and THC combined can produce severe performance impairment even when given at low doses. The combined effect of alcohol and cannabis on performance and crash risk appeared additive in nature, i.e. the effects of alcohol and cannabis combined were always comparable to the sum of the effects of alcohol and THC when given alone.”

Williams et al. 1985. Drugs in fatally injured young male drivers. Public Health Reports 1: 19-26.

Ramaekers et al. 2004. op. cit.

US Department of Justice, Bureau of Justice Statistics. op. cit.

US Department of Health and Human Services, Substance and Mental Health Services Association, Office of Applied Studies. 1998. op. cit.

Canadian Public Health Association. “The Pot and Driving Campaign.”

US Government Accountability Office. ONDCP Media Campaign: Contractor's National Evaluation Did Not Find that the Youth Anti-Drug Media Campaign Was Effective in Reducing Youth Drug Use: Report to the Subcommittee on Transportation, Treasury, the Judiciary, Housing and Urban Development, and Related Agencies, Committee on Appropriations, U.S. Senate. August 25, 2006.

Papafotiou et al. 2005. An evaluation of the sensitivity of the Standardised Field Sobriety Tests (SFSTs) to detect impairment due to marijuana intoxication. Psychopharmacology 180: 107-114.

Khiabani et al. 2006. Relationship between THC concentration in blood and impairment in apprehended drivers. Traffic Injury Prevention 7: 111-116.

Looby et al. 2007. Roadside sobriety tests and attitudes toward a regulated cannabis market. Harm Reduction Journal. Online document accessed November 24, 2007




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Fitness to drive in spite (because) of THC

Strohbeck-Kühner P, Skopp G, Mattern R Arch Kriminol 2007 Jul-Aug; 220(1-2):11-9.

Attention-deficit/hyperactivity disorder (ADHD) is characterized by a lack of concentration and/or an altered activation level. People with ADHD are found to violate traffic regulations, to commit criminal offences and to be involved in traffic accidents more often than the statistical norm. Furthermore, they show more deviant behaviour and have an increased co-morbidity regarding substance abuse and dependence.

Hence, this disorder is of some forensic importance. The purpose of this case study is to demonstrate that in some cases people with ADHD may show unusual effects after the consumption of THC. A 28-year-old male, who showed abnormal behaviour and seemed to be significantly maladjusted and inattentive while sober, appeared to be completely normal with a very high plasma level of THC.

Performance tests conducted with the test batteries ART2020 and TAP provided average and partly above-average results in functions related to driving. Thus, it has to be taken into account that in persons with ADHD THC may have atypical and even performance-enhancing effects.




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Drivers With THC in their Blood Have Only a Small Increased Risk to Cause an Accident

Researchers of the University of Wroclaw, Poland, investigated the effects of an ointment with structured physiological lipids and endocannabinoids in 21 patients with pruritus due to end-stage failure of kidney function. So-called uremic pruritus is still a common symptom in patients with end-stage renal failure. However, there is no effective treatment for this condition. All patients applied the tested cream twice daily for a period of three weeks. Global pruritus and dry skin were examined before the trial, on study visits at weekly intervals, and two weeks after completion of the study.

After 3-week therapy pruritus was completely eliminated in 8 patients. Dry skin was significantly improved. Researchers noted that "it is very probable that the observed decrease of pruritus with the test product therapy was not only the result of dry skin improvement but that the addition of endocannabinoids may have also played a role."

(Source: Szepietowski JC, Szepietowski T, Reich A. Efficacy and tolerance of the cream containing structured physiological lipids with endocannabinoids in the treatment of uremic pruritus: a preliminary study. Acta Dermatovenerol Croat 2005;13(2):97-103.)




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The effect of cannabis compared with alcohol on driving

Sewell RA, Poling J, Sofuoglu M

Am J Addict 2009 May-Jun; 18(3):185-93.

The prevalence of both alcohol and cannabis use and the high morbidity associated with motor vehicle crashes has lead to a plethora of research on the link between the two. Drunk drivers are involved in 25% of motor vehicle fatalities, and many accidents involve drivers who test positive for cannabis.

Cannabis and alcohol acutely impair several driving-related skills in a dose-related fashion, but the effects of cannabis vary more between individuals than they do with alcohol because of tolerance, differences in smoking technique, and different absorptions of Delta(9)-tetrahydrocannabinol (THC), the active ingredient in marijuana.

Detrimental effects of cannabis use vary in a dose-related fashion, and are more pronounced with highly automatic driving functions than with more complex tasks that require conscious control, whereas alcohol produces an opposite pattern of impairment. Because of both this and an increased awareness that they are impaired, marijuana smokers tend to compensate effectively while driving by utilizing a variety of behavioral strategies.

Combining marijuana with alcohol eliminates the ability to use such strategies effectively, however, and results in impairment even at doses which would be insignificant were they of either drug alone. Epidemiological studies have been inconclusive regarding whether cannabis use causes an increased risk of accidents; in contrast, unanimity exists that alcohol use increases crash risk.

Furthermore, the risk from driving under the influence of both alcohol and cannabis is greater than the risk of driving under the influence of either alone. Future research should focus on resolving contradictions posed by previous studies, and patients who smoke cannabis should be counseled to wait several hours before driving, and avoid combining the two drugs.



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Abstracts of several studies

Cimbura G, Lucas DM, Bennett RC, Warren RA, Simpson HM
Incidence and toxicological aspects of drugs detected in 484 fatally injured drivers and pedestrians in Ontario.
Journal of Forensic Sciences
1982 Oct
Results are presented of a comprehensive drug study carried out on specimens from drivers and pedestrians fatally injured in Ontario. Toxicological analyses were regularly performed on blood and urine and occasionally on vitreous humor, stomach contents, and liver.
The analytical procedures could detect and quantitate a wide variety of drugs including such illicit drugs as Cannabis. With respect to drivers, alcohol was found in 57% of the study sample and drugs other than alcohol, in 26%.
However, in only 9.5% of the drivers were psychoactive drugs (other than alcohol) detected in the blood in concentrations that may adversely affect driving skills. Delta-9-Tetrahydrocannabinol and diazepam accounted for a majority of the findings in this category.


Dackis CA, Pottash AL, Annitto W, Gold MS
Persistence of urinary marijuana levels after supervised abstinence.
American Journal of Psychiatry
1982 Sep
The authors present a case report of the presence of urinary cannabinoids during 21 days of supervised abstinence from chronic marijuana use and provide data on 6 similar cases. They discuss the theoretical implications of the persistence of cannabinoids.


Mason AP, McBay AJ
Ethanol, marijuana, and other drug use in 600 drivers killed in single-vehicle crashes in North Carolina, 1978-1981.
Journal of Forensic Sciences
1984 Oct
Although the use of ethanol, marijuana, and other drugs may be detrimental to driving safety, this has been established by direct epidemiological evidence only for ethanol. In this study, the incidences of detection of ethanol (and other volatile substances), delta-9-tetrahydrocannabinol (THC), barbiturates, cocaine and benzoylecgonine, opiates, and phencyclidine were determined in an inclusive population of 600 verified single-vehicle operator fatalities that occurred in North Carolina in 1978 to 1981.
The incidence of detection of amphetamines and methaqualone were determined for drivers accepted for study during the first two years (n = 340) and the last year (n = 260), respectively. Blood concentrations of 11-nor-deta-9-tetrahydrocannabinol-9-carboxylic acid (9-carboxy-THC) were determined in THC positive drivers. EMIT cannabinoid assays were performed on blood specimens from all drivers accepted for study during the third year, and the feasibility of using the EMIT cannabinoid assay as a screening method for cannabinoids in forensic blood specimens was investigated. The incidence of detection of ethanol (79.3%) was far greater than the incidences determined for THC (7.8%), methaqualone (6.2%), and barbiturates (3.0%).
Other drugs were detected rarely, or were not detected. Blood ethanol concentrations (BECs) were usually high; 85.5% of the drivers whose bloods contained ethanol and 67.8% of all drivers had BECs greater than or equal to 1.0 g/L. Drug concentrations were usually within or were below accepted therapeutic or active ranges. Only a small number of drivers could have been impaired by drugs, and most of them had high BECs. Multiple drug use (discounting ethanol) was comparatively rare. Ethanol was the only drug tested for that appears to have a significantly adverse effect on driving safety.


Gjerde H, Kinn G
Impairment in drivers due to cannabis in combination with other drugs.
Forensic Science International
1991 Jul-Aug
Blood samples from 425 suspected drugged drivers who were clinically impaired and negative for alcohol were analysed. Fifty-six percent of the samples were positive for tetrahydrocannabinol (THC). Tetrahydrocannabinol-positive blood samples were analysed for amphetamines, barbiturates, benzodiazepines, cocaine metabolites and opiates. Eighty-two percent of the samples were found to be positive for one or more drugs in addition to THC, and the concentrations of these drugs were often high. Thus, THC in combination with other drugs seems to be a much more frequent reason for impairment than THC alone among Norwegian drugged drivers.


Id Code
Cimbura G, Lucas DM, Bennett RC, Donelson AC
Incidence and toxicological aspects of cannabis and ethanol detected in 1394 fatally injured drivers and pedestrians in Ontario (1982-1984).
Journal of Forensic Sciences
1990 Sep
A comprehensive epidemiological study of the involvement of cannabis and ethanol in motor vehicle fatalities in the Province of Ontario, Canada, is described. The study is based on toxicological analyses of blood and, when available, urine specimens. Ethanol was determined by headspace gas chromatography (GC). For cannabis, the methods employed were radioimmunoassays (RIAs) for screening and gas chromatography/mass spectrometry (GC/MS) for the determination of delta-9-tetrahydrocannabinol (THC) in blood. The study sample consisted of 1169 drivers and 225 pedestrians. THC was detected in the blood of 127 driver victims (10.9%) in concentrations ranging from 0.2 to 37 ng/mL, with a mean of 3.1 +/- 5.0 ng/mL. Ethanol was found in 667 driver victims (57.1%), in concentrations ranging from 9 to 441 mg/100 mL, with a mean of 165.8 +/- 79.5 mg/100 mL. For pedestrians, the incidence of THC and ethanol in the blood was 7.6 and 53.3%, respectively. The incidence of THC in the driver victims in this study constitutes an approximately threefold increase over the results of an Ontario study completed in 1979. At least a part of the increase may be attributed to interstudy differences in analytical methodology for cannabinoids.


Id Code
Marks DF, MacAvoy MG
Divided attention performance in cannabis users and non-users following alcohol and cannabis separately and in combination.
The effect of delta 9-tetrahydrocannabinol (delta 9-THC) and alcohol, singly and in combination, on divided attention performance was investigated in cannabis users and non-users who were matched for alcohol use. Both cannabis and alcohol produced decrements in central and peripheral signal detections. Drug and alcohol effects were greater for signal presentations in the periphery. Cannabis users were less impaired in peripheral signal detection than non-users while intoxicated by cannabis and/or alcohol. These findings suggest the development of tolerance and cross-tolerance in regular cannabis users and/or the ability to compensate for intoxication effects.


Id Code
Budd RD, Muto JJ, Wong JK
Drugs of abuse found in fatally injured drivers in Los Angeles County.
Drug & Alcohol Dependence
1989 Apr
Blood or urine specimens from nearly 600 fatally injured drivers in two Los Angeles County studies were analyzed for the presence of alcohol and other drugs of abuse, including PCP, cocaine, opiates and marijuana. The results of the preliminary study indicate that 65 out of 102 fatally injured drivers had used alcohol and/or another drug of abuse - 34 had used alcohol only, 12 had used one or more other drug(s) of abuse, and 19 had used alcohol in combination with another drug of abuse. The results of the larger follow-up study, begun a year later, indicate a continued high level of both alcohol use (41.5%) and marijuana use (19%) with moderate cocaine usage (8%) and low levels (less than 2%) of barbiturate and PCP usage.


Id Code
Kielland KB
[Urinary excretion of cannabis metabolites].
Tidsskrift for Den Norske Laegeforening
1992 May 10
Urine testing is increasingly used to detect drug abuse, most commonly by easily performed immunological tests. There is large interpersonal variation in the excretion time of cannabinoids. Excretion times of up to 11 weeks have been demonstrated. In cases with a long excretion time a negative test result may be followed by a positive result without concomitant abuse. We describe a case where cannabinoid metabolites in urine were detected by a routine immunological method (Abbotts ADx) after 95 days of supervised abstinence. It is important that personnel evaluating test results have a thorough knowledge of possible pitfalls.



- Logan BK, Schwilke EW
- Drug and alcohol use in fatally injured drivers in Washington State.
- Eng
- 1996 May
- 0022-1198
- J Forensic Sci
- 505-10
- Blood and/or urine from fatally injured drivers in Washington State were collected and tested for the presence of drugs and alcohol. Drug and/or alcohol use was a factor in 52% of all fatalities. Among single vehicle accidents, alcohol use was a factor in 61% of cases versus 30% for multiple vehicle accidents. Drugs most commonly encountered were marijuana (11%), cocaine (3%), amphetamines (2%), together with a variety of depressant prescription medications.
Trends noted included an association of depressant use with higher blood alcohol levels, while marijuana use was associated with lower blood alcohol levels. Marijuana use was noted to be most prominent in the 15-30 year age group, stimulant use in the 21-40 year old group, and prescription depressant use was more prevelant in the 45 + age group. Drug use demographics in this population are consistent with those noted in other jurisdictions.
Research Institute
- Department of Laboratory Medicine, University of Washington, Seattle, 98134-2027, USA.
- J Forensic Sci 1996 May;41(3):505-10


- Sugrue M, Seger M, Dredge G, Davies DJ, Ieraci S, Bauman A, Deane SA, Sloane D
- Evaluation of the prevalence of drug and alcohol abuse in motor vehicle trauma in south western Sydney.
- Eng
- 1995 Dec
- 0004-8682
- Aust N Z J Surg
- 853-6
- This study estimated prospectively the prevalence of high drug and alcohol levels in road trauma cases who met the criteria for activation of the Liverpool Hospital's trauma team. Urine analysis of road trauma victims between October 1992 and October 1993 was undertaken for drug and alcohol estimation.
A total of 164 drivers were studied. A urine alcohol concentration (UAC) exceeding 0.08 g/dL was detected in 27 drivers (16.5%). Cannabinoids were detected in the urine of 25 drivers (15.2%), in 17 the concentrations exceeded 400 ng/mL. In one instance amphetamine, cocaine and heroin were detected in the same injured driver.
Combined use of alcohol with some other drugs was detected in only four drivers. Alcohol and cannabinoid levels were prevalent in the urine of injured drivers in this study, particularly in young males who remain over-represented in the group of injured drivers. In the population surveyed other drugs were rarely detected. The role of cannabinoids in road trauma and the use of cannabinoids in young male drivers will however need to be monitored more extensively.
Research Institute
- Department of Trauma, Liverpool Hospital, Sydney, New South Wales, Australia.
- Aust N Z J Surg 1995 Dec;65(12):853-6


- Marquet P, Delpla PA, Kerguelen S, Bremond J, Facy F, Garnier M, Guery B, Lhermitte M, Mathe D, Pelissier AL, Renaudeau C, Vest P, Seguela JP
- Prevalence of drugs of abuse in urine of drivers involved in road accidents in France: a collaborative study.
- Eng
- 1998 Jul
- 0022-1198
- J Forensic Sci
- 806-11
- The collaborative, anonymous, case-control study was intended to determine the prevalence of opiates, cocaine metabolites, cannabinoids and amphetamines in the urine of drivers injured in road accidents and to compare these values with those of non-accident subjects ("patients") in France. Recruitment was performed nationwide in the emergency departments of five hospitals and comprised 296 "drivers" aged 18 to 35 and 278 non-traumatic "patients" in the same age range.
Females represented 28.4% of "drivers" and 44.2% of "patients." Screening for drugs in urine was performed by fluorescence polarization immunoassays in each center. Each positive result was verified using gas chromatography-mass spectrometry (GC-MS), in a single laboratory. Statistical analysis comprised single-step logistic regression and simultaneously took account of confounding factors and the final differences in prevalence values between the two populations or different subgroups.
Cannabinoids were found in 13.9% of drivers (16.0% of males and 8.3% of females, p < 0.05) and 7.5% of patients (12.3% of males, 1.6% of females, p < 0.0001); only in females was this prevalence higher in injured drivers than in patients (p < 0.05).
Opiates were present in 10.5% of drivers' and 10.4% of patients' urine samples (NS), and were more frequent in urine samples positive for cannabinoids, in drivers (p < 0.01) as well as in patients (p < 0.001). The prevalence of cocaine metabolites in drivers and patients was 1.0 and 1.1% and that of amphetamines 1.4 and 2.5%, respectively.
No causal relationship between drugs and accidents should be inferred from this retrospective study. Nevertheless, the high prevalence of cannabis and opiate (licit or illicit) use in young people, whether injured drivers or patients, has potential implications for road traffic safety in France. Cocaine and amphetamines did not appear to be a major problem, unlike the experience in other countries.
Research Institute
- Department of Pharmacology-Toxicology and Emergency Care Unit, University Hospital, Limoges, France.
- J Forensic Sci 1998 Jul;43(4):806-11


- Costantino A, Schwartz RH, Kaplan P
- Hemp oil ingestion causes positive urine tests for delta 9- tetrahydrocannabinol carboxylic acid.
- Eng
- 1997 Oct
- 0146-4760
- J Anal Toxicol
- 482-5
- A hemp oil product (Hemp Liquid Gold) was purchased from a specialty food store. Fifteen milliliters was consumed by seven adult volunteers. Urine samples were taken from the subjects before ingestion and at 8, 24, and 48 h after the dose was taken. All specimens were screened by enzyme immunoassay with SYVA EMIT II THC 20, THC 50, and THC 100 kits. The tetrahydrocannabinol carboxylic acid (THCA) concentration was determined on all samples by gas chromatography-mass spectrometry (GC- MS) (5). A total of 18 postingestion samples were submitted.
Fourteen of the samples screened above the 20-ng cutoff, seven were above the 50- ng cutoff, and two screened greater than the 100-ng cutoff. All of the postingestion samples showed the presence of THCA by GC-MS.
Research Institute
- American Medical Laboratory, Chantilly, Virginia 20151, USA.
- J Anal Toxicol 1997 Oct;21(6):482-5


- Fortner N, Fogerson R, Lindman D, Iversen T, Armbruster D
- Marijuana-positive urine test results from consumption of hemp seeds in food products.
- Eng
- 1997 Oct
- 0146-4760
- J Anal Toxicol
- 476-81
- Commercially available snack bars and other foodstuffs prepared from pressed hemp seeds were ingested by volunteers. Urine specimens were collected for 24 h after ingestion of the foodstuffs containing hemp seeds and tested for marijuana using an EMIT immunoassay and gas chromatography-mass spectrometry (GC-MS). Specimens from individuals who ate one hemp seed bar demonstrated little marijuana immunoreactivity, and only one specimen screened positive at a 20-ng/mL cutoff.
Specimens from individuals who ate two hemp seed bars showed increased immunoreactivity, and five specimens screened positive at a 20-ng/mL cutoff. A single specimen yielded a quantitative GC-MS value (0.6 ng/mL), but it failed to meet reporting criteria. Several specimens from individuals who ate three cookies made from hemp seed flour and butter screened positive at both 50- and 20-ng/mL cutoffs.
Two specimens produced quantitative GC-MS values (0.7 and 3.1 ng/mL), but they failed to meet reporting criteria. Several specimens also tested positive with an FDA-approved on-site marijuana-screening device. Hemp seeds similar to those used in the foodstuffs did not demonstrate the presence of marijuana when tested by GC-MS. In this study, ingestion of hemp seed food products resulted in urine specimens that screened positive for marijuana. No specimens gave a GC-MS quantitative value above the limit of detection for marijuana.
Research Institute
- PharmChem Laboratories, Inc., Menlo Park, California 94025, USA.
- J Anal Toxicol 1997 Oct;21(6):476-81


- Lehmann T, Sager F, Brenneisen R
- Excretion of cannabinoids in urine after ingestion of cannabis seed oil [see comments]
- Eng
- 1997 Sep
- 0146-4760
- J Anal Toxicol
- 373-5
- Gas chromatographic-mass spectrometric (GC-MS) quantitation of 25 cannabis sed oils determined delta 9-tetrahydrocannabinol (THC) concentrations from 3 to 1500 micrograms/g oil. In a pilot study, the morning urine of six volunteers who had ingested 11 or 22 g of the oil, which contained the highest THC content (1500 micrograms/g), was collected for six days. The urine samples were screened by immunoassay, and the content of 11-nor-9-carboxy-delta 9-THC (THCCOOH) was determined by GC-MS. Urine samples were found cannabis positive for up to six days with THCCOOH-equivalent concentrations up to 243 ng/mL. by the Abuscreen OnLine immunoassay and THCCOOH contents from 5 to 431 ng/mL by the GC-MS method. All subjects reported THC-specific psychotropic effects.
Research Institute
- Institute of Pharmacy, University of Bern, Switzerland.
- Comment in: J Anal Toxicol 1998 Jan-Feb;22(1):80-1
- J Anal Toxicol 1997 Sep;21(5):373-5


- Struempler RE, Nelson G, Urry FM
- A positive cannabinoids workplace drug test following the ingestion of commercially available hemp seed oil [see comments]
- Eng
- 1997 Jul-Aug
- 0146-4760
- J Anal Toxicol
- 283-5
- A commercially available health food product of cold-pressed hemp seed oil ingested by one volunteer twice a day for 4 1/2 days (135 mL total). Urine specimens collected from the volunteer were subjected to standard workplace urine drug testing procedures, and the following concentrations of 11-nor-delta9- tetrahydrocannabinol carboxylic acid (9-THCA) were detected: 41 ng/mL 9-THCA at 45 h, 49 ng/mL at 69 h, and 55 ng/mL at 93 h.
Ingestion was discontinued after 93 h, and the following concentrations were detected: 68 ng/mL at 108 h, 57 ng/mL at 117 h, 31 ng/mL at 126 h, and 20 ng/mL at 142 h. The first specimen that tested negative (50 ng/mL initial immunoassay test, 15 ng/mL confirmatory gas chromatographic-mass spectrometric test) was at 146 h, which was 53 h after the last hemp seed oil ingestion. Four subsequent specimens taken to 177 h were also negative.
This study indicates that a workplace urine drug test positive for cannabinoids may arise from the consumption of commercially available cold-pressed hemp seed oil.
Research Institute
- ARUP Laboratories, Inc., Salt Lake City, Utah 84108, USA.
- Comment in: J Anal Toxicol 1998 Jan-Feb;22(1):80-1
- J Anal Toxicol 1997 Jul-Aug;21(4):283-5
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