Cannabis and Driving?
What do the experts say?


from HEMP SA article on driving and Cannabis
This page includes references to several good articles about Cannabis & driving as well as a message from one of Australia's leading scientists.

Marijuana safer than drink, says crash study, The Advertiser

If alcohol could be replaced by marijuana it would be a major road safety advance, one of Australia's leading road safety specialists claims...

Cannabis cleared in report on road toll, The Australian (undated)
Motorists who use Cannabis are no more likely to be involved in a fatal collision than drivers who are drug free, a study by the Victorian Institute of Forensic Pathology says...

Drink, not Cannabis causes road deaths LTE by Dr Alex Wodak, The Australian

Marijuana's Effects on Actual Driving Performance by HWJ Robbe, Institute for Human Psychopharmacology, University of Maastricht, The Netherlands
(A paper presented at the 13th International Conference on Alcohol, Drugs & Traffic Safety, Adelaide SA. )

Cannabis & Driving by Dr Greg Chesher


I want to mention the present understanding about Cannabis and driving. You may already be quite familiar with these studies, but in case you are not, the following might interest you. The law about alcohol which proscribes driving with BAC >0.05 g% blood alcohol has been determined by epidemiological studies employing the case-control method. At 0.05g% the driver is about twice as likely to be involved in a crash as a driver with zero BAC. As the BAC increases, so in a very steep fashion, does the probability of being involved in a crash. The role of alcohol in road crashes has been very clearly demonstrated. The drink-driving laws are based upon sound science!

As for other drugs, including Cannabis, the pharmacokintetics do not permit this technique. There is no correlation between the blood concentration of Cannabinoids and impairment as determined on laboratory tasks. Furthermore, the study would require the taking of blood from the crash involved drivers as well as that of the control drivers--not involved in a crash. There is no equivalent to the alcohol breathalyser. Cannabinoids are not excreted on the breath. (It is the collection of adequate data from a control group that really precludes the case-control method). So to overcome this, the technique of "culpability analysis" has been employed.

In this case, data for crashes are presented to an independent group of observers who are ignorant (blind) as to whether any of the drivers had any drug at all in their blood. By studying the information of the crash (events before and up to the crash) a score apportioning the degree of blame to each driver is given. From this "culpability" is determined.

So far there have been four studies using this technique, two in the USA and two in Australia.

As far as Cannabis is concerned, it is quite remarkable that in all cases the results were so similar as to be the same finding. The culpability ratio for the drivers bearing Cannabinoids in blood revealed that they were no more likely to be considered as a cause of the crash as those who had no drugs at all in their blood. Indeed in each of these four studies the Cannabinoid drivers were less (but not significantly so) likely to have been a cause than those with no drug at all. In all of these studies alcohol-bearing drivers were overwhelmingly culpable.

So at the moment, the present data suggest that cannabis is not involved as a causative factor in road crashes.

Two things must be considered. First, it is early days so far. The total number of cases in all of these studies totals about 9000 or a little less.

Second, the determination of the culpability score is dependent upon the integrity of the initial data describing the accident. In most cases this is from the attending police. This is not always reliable. However, as the numbers increase with more studies, we will have more confidence with the findings.

There is another Australian study currently being analysed and is due to be released shortly.....SOON is the advice I have from the author.


Greg Chesher



Drummer, O. (1994). Drugs and drivers killed in Australian Road traffic accidents. The use of responsibility analysis to investigate the contribution of drugs to fatal accidents: Victorian Institute of Forensic Pathology. Monash University.

Hunter, C., Lokan, R., Longo, M., White, J. & White, M. (1998). The Prevalence and Role of Alcohol, Cannabinoids, Benzodiazepines and stimulants in Non-Fatal Crashes. Adelaide: Forensic Science, Department for Administrative and Information Services, South Australia.

Terhune, K., Ippolito, C., Hendricks, D., Michalovic, J., Bogema, S., Santinga, P., Blomberg, R. & Preusser, D. (1992). The incidence and role of drugs in fatally injured drivers.: US Department of Transportation National Highway Traffic Safety Administration.

Williams, A., Peat, M., Crouch, D., Wells, J. & Finkle, B. (1985). Drugs in fatally injured young male drivers. Public Health Reports, 100, 19-25

Dr Gregory B. Chesher



A single glass of wine will impair your driving more than smoking a joint. And under certain test conditions, the complex way alcohol and cannabis combine to affect driving behaviour suggests that someone who has taken both may drive less recklessly than a person who is simply drunk.

These are the findings of a major new study by British transport researchers. The unpublished research, seen exclusively by New Scientist, stops well short of condoning driving under the influence of even small amounts of cannabis. But in a week which has seen renewed debate in Britain surrounding the criminalisation of cannabis, it throws an uncomfortable spotlight on a problem confronting governments everywhere - how to deter the growing numbers of cannabis users from "dope driving".

At present there is no accurate test that can reveal whether a driver has taken cannabis before driving, and developing one will not be easy. But even when this problem is cracked, another will remain - where to set the safety threshold for smoking cannabis.

Advocates of zero tolerance say there should be penalties for drivers caught with any amount of recently smoked cannabis in their body. The new research suggests that would only be credible if governments also adopted zero tolerance on drink driving.

Middle Of The Road

The new study was undertaken by the Transport Research Laboratory in Crowthorne, Berkshire, and confirms the results of a preliminary study more than a year ago. Researchers at the TRL, led by Barry Sexton, gave 15 volunteers doses of cannabis or alcohol, or a combination of both, before letting them loose on an array of psychomotor tests and a sophisticated driving simulator.

The volunteers were given either enough alcohol to raise alcohol levels in the blood to 50 milligrams per 100 millilitres - about 60 per cent of Britain's legal limit of 80 mg/100 ml - or a specially prepared marijuana joint designed to deliver the same high typically experienced by smokers.

In the study, cannabis significantly affected only one criterion, known as tracking ability. Volunteers found it more difficult to hold a constant speed and follow the middle of the road accurately while driving around a figure-of-eight loop. The TRL researchers point out in their draft report that this test requires drivers to hold their concentration for a short time, a task which is particularly badly affected by the intoxicating effects of cannabis.

Cautious Driving

However, volunteers drinking the equivalent of a glass of wine fared worse than those who had smoked a joint. Those who were given both alcohol and cannabis performed worse still, reinforcing the idea that alcohol has a cumulative effect when taken with other drugs.

But the study also found that drivers on cannabis tended to be aware of their intoxicated state, and drove more cautiously to compensate. Indeed, doped-up volunteers often rated themselves as being more impaired than police surgeons brought in to evaluate their sobriety.

Surprisingly, drinking alcohol didn't offset this cautious behaviour, opening up the unproven possibility that a driver who is moderately drunk might be better off under some conditions if they had also smoked.

This cautious behaviour is in line with findings by other researchers. "Whereas alcohol promotes risk taking like fast speeds and close following, cannabis promotes conservative driving, but may cause attention problems and misperceptions of time," says Nicholas Ward, technical adviser to the Immortal project - a three-year European Union trial designed to quantify the crash risk drivers face after taking various drugs and medicines.


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


Risk analysis studies to investigate the contribution of drugs to accident causation are limited. We have used a method based on establishing the responsibility of a driver to investigate the involvement of drugs other than alcohol in 1052 fatally injured drivers. The proportion of drivers deemed to be responsible in a drug-free group were compared to drivers with target drugs found in their blood stream.

Drugs (including alcohol) were detected in 49% of the drivers. Alcohol was detected in 36% of the cases, whilst drugs were detected in 22%. 13% had only drugs detected. The remaining 9% of the population involved a combination of drugs and alcohol. The order of prevalence of drugs were marijuana (112 cases), amphetamines and related stimulants (35), benzodiazepines (34) and opiates (34).

Drivers in whom only opiates were detected had an odd's ratio of 2.4, whilst marijuana cases provided a relative risk of 0.6. Drivers in whom stimulants were detected gave an odd's ratio of 1.4 whilst benzodiazepines gave an odd's ratio of 1.0. By contrast the odd's ratio for alcohol was 6.8.

Drivers with higher than therapeutic concentrations detected represented 22 drivers (2.1%). Most of these drivers were found to be culpable. Multiple drug cases also tended to be culpable. The culpability rate in this group was 89% compared to 70% in drug-free drivers.

These data show that only a small proportion of impaired drivers are drug effected, the remainder being impaired by alcohol. The relative risk for psychoactive drugs is also not uniform, with marijuana use providing the least effect on risk, whilst opiate use seems to provide the largest increase in risk compared to the other drug groups studied.


While certain psychoactive drugs other than alcohol can adversely affect driving skills in simulated studies, epidemiological risk analysis data which provides an assessment of the contribution, of specific drugs or drug classes to accident causation is limited. The effect of benzodiazepines and minor tranquillisers on accident risk are uncertain [Skegg et al, 1979; Jick et al, 1981; Lagier et al, 1993], while the effects of THC appears to be slight or even protective [Terhune et al, 1992; Williams et al, 1985]. Assessment of risk for other drugs or drug groups has not been studied in any detail.

We have used a method based on establishing the responsibility of a driver using strict scoring guidelines [Robertson & Drummer, 1994]. We have used this approach to investigate the involvement of drugs in over 1000 fatal accidents.


Information of drivers killed in motor vehicle accidents were obtained from records kept either at the Victorian Institute of Forensic Pathology and the State Coroner's Office (Victoria), numerous Coroner's courts in NSW or the Western Australian Police (WA). Accidents occurred from January 1990 to December 1993.

Drivers were scored for responsibility as described by Robertson and Drummer [1994]. Cases in which insufficient information was available to allow an assessment of culpability were omitted from the analysis. Toxicology data relating to drivers scored for responsibility were only assessed after the responsibility analysis was conducted.

Toxicological analysis was conducted for a large range of drugs including the drugs of abuse, the benzodiazepines and other prescription drugs. Drugs administered in hospital were excluded. Only cases which had both alcohol and a full range of drug tests were used in the study.

The proportion of drivers deemed to be responsible in a drug-free group were compared to drivers with target drugs found in their blood stream. Accident risk is defined as the odd's ratio of a drug group over the drug-free control group.

Means and 95% confidence intervals are shown in the text and Tables. Data were analysed by smoothed logistic regression. Odd's ratios were adjusted for age and sex [Schleselman, 1982]. EGRET statistical software was used to evaluate these data.


Drugs (including alcohol) were detected in 49% of the drivers. Alcohol was detected in 36% of the cases, whilst drugs were detected in 22%. 13% had only drugs detected. The remaining 9% of the population involved a combination of drugs and alcohol.

The most common drugs detected are shown in Table 1. Over forty drugs were represented in these cases. Illegal drugs were found in 12.9% of the drivers. The most common illegal drugs were marijuana and the stimulants (This group represents the amphetamines, the ephedrines and the slimming drugs such as phentermine etc). However four cases involved heroin and only one case involved cocaine.

Table 1
Type and Abundance of Drugs

DrugAbundance (%)
Anti-depressant drugs1.0
Anti-convulsant drugs0.6
Anti-inflammatory drugs0.6

Odd's ratios and statistical significance for drug groups are shown in Table 2. Drivers in whom opiates, stimulants or benzodiazepines were detected gave odd's ratio of 2.0 compared to an age and sex matched drug free group, but neither drug group were significant statistically. Marijuana cases provided an odd's ratio of 0.6, but again this was not significant statistically. By contrast the odd's ratio for alcohol was 7.6.

Table 2
Adjusted Odd's Ratios for Selected Drugs

DrugOdd's Ratio*P-value
Alcohol7.6 (4.6-12)<0.001
Any Drug1.4 (0.9-2.2)0.130
Cannabis0.6 (0.3-1.0)0.065
Stimulants2.0 (0.7-5.6)0.217
Benzodiazepines2.0 (0.6-7.0)0.295
Opiates2.0 (0.7-6.3)0.220
Misc. Drugs2.8 (1.1-7.3)0.040

* 95% confidence interval in parentheses

Drivers with higher than therapeutic concentrations detected represented 22 drivers (2.1%). Most of these drivers were found to be responsible. Multiple drug cases (alcohol excluded) were found in 24 cases (2.3%). Most of these cases were responsible. For example, of the 24 drivers in whom more than one psychoactive drug was detected (alcohol excluded), 22 were deemed culpable, 2 were contributory and 2 were not culpable.


The use of responsibility rates of drivers has been used to establish that alcohol increases accident risk [Terhune et al, 1992; Williams et al, 1985]. This is again confirmed in this study. The prevalence of alcohol at over 30% in the Australian fatally-injured driver population reinforces the magnitude of alcohol-related trauma which still exist on Australian roads.

The contribution of drug use on road trauma and accident risk is less well defined. While most common drug type detected were the illegal drugs, marijuana, certain stimulants and certain opiates, their effect individually and collectively were small compared to alcohol.

Drivers with higher than therapeutic drug concentrations and drivers involving multiple drug use tended to be responsible. If cases involving alcohol were excluded, then such cases represented 2.6% of the driving population. This contrasts to ~30% of the population involving alcohol over 0.05 gram/100 mL (general legal limit in Australia).

It was of some interest that cannabis tended to show a negative effect on relative risk when other drug groups showed an increase. This phenomenon has also been seen elsewhere [Terhune et al, 1992; Williams et al, 1985]. The most likley reason probably relates to the over compensation of marijuana-using drivers on their driving skills. Over compensation may be caused simply by slowing down and avoiding adverse driving situations. These observations do not seem to be related to whether delta-9-THC or 11-carboxy-THC are measured in blood [Terhune et al, 1992; Williams et al, 1985].

In conclusion, these findings show that the contribution of drugs to accident causation is much lower than for alcohol. While more cases are required before any definitive conclusions can be made on the effect of specific drug types on driving risk, cases involving multiple drug use and higher than therapeutic drug concentrations tended to be culpable in fatal accidents.


There were many persons and/or organisations who provided valuable assistance to various parts of this project;. Ms. Lynette Kornmehl; Ms Maryanne Maisey; the staff of the Victorian Institute of Forensic Pathology and the State Coroners Office; Dr. Phillip Swann of Vic Roads; members of the Advisory Group on Drugs and Driving (AGODD); Dr. Peter Vulcan of the Accident Research Centre, Monash University; NSW State Coroners; staff of the NSW Institute of Forensic Medicine; staff of the NSW Division of Analytical Laboratories; Dr. David Saffron of the Road Safety Unit of the NSW Roads and Traffic Authority; the Federal Office of Road Safety; WA Police; the WA Coroner; staff of the Toxicology Section of the Western Australian Department of Mines, and Professor John McNeil and Dr. Andrew Forbes of the Department of Social and Preventative Medicine, Monash University.


H. Jick, J. R. Hunter, B. J. Dinan, S. Madsen and A. Stergachis. Sedating drugs and automobile accidents leading to hospitalization. Am. J. Pub. Health 7: 1399-1400 (1981).

Lagier et al, Benzodiazepine/Driving Collaborative Group. Are benzodiazepines a risk factor for road accidents. Drug and Alcohol Dependence, 33: 19-23 (1993).

M. D. Robertson and O. H. Drummer. A methodology to study the effect of drugs in driving. Accid. Anal. and Prev. 26: 243-7 (1994).

J. J. Schleselman. In: "Case-control studies". New York Oxford University Press (1982).

D. C. G. Skegg, S. M. Richards and R. Doll. Minor tranquillisers and road accidents. Br. Med. J. 1: 917-9 (1979).

K.W. Terhune, C. A. Ippolito, D. L. Hendricks, J. G. Michalovic, S. C. Bogema, P. Santinga, R. Blomberg and D. F. Preusser . The incidence and role of drugs in fatally injured drivers. US Department of Transportation, National Highway Traffic Safety Administration, Report DOT HS 808 065; (1992).

A. F. Williams, M. J. Peat, D. J. Crouch, J. K. Wells and B. S. Finkle. Drugs in fatally injured young male drivers. Public Health Reports. 100: 19-25 (1985).