Own your ow legal marijuana business
Your guide to making money in the multi-billion dollar marijuana industry
Miscellaneous Statements on Drug Policy
References on Drugs and Driving


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.

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. 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 g/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.
Library Highlights

Drug Information Articles

Drug Rehab