Moskowitz (1985) reviewed a large number of studies on the effects of marijuana on psychological abilities. In reaction time experiments neither the speed of initial detection nor the speed of responding appears to be affected by marijuana, although the frequently reported increase of RT variability suggests that attentional mechanisms are impaired by marijuana. Tracking is significantly affected by marijuana. Also, perceptual functions and vigilance are negatively affected by this drug.
However, based on a review of a number of epidemiological studies, Moskowitz (1985) concluded that there is little evidence for an increased risk of accident involvement under marijuana. Robbe (1994)reviewed the epidemiological literature as well and concluded that some people do drive after cannabis use and that drivers involved in accidents often show the drug’s presence. However, because alcohol has been a severe confounding factor in all surveys of accident-involved drivers, the independent contribution of marijuana to accidents remains unclear.
The effects of marijuana on driving behaviour has been examined in many experiments. According to Robbe (1994), the foremost impression one gains from reviewing the literature is that no clear relationship has been demonstrated between marijuana and either seriously impaired driving performance or the risk of accident involvement. Smiley (1986) compared simulator and on-road studies of marijuana effects on car driving performance. In simulator studies with realistic car dynamics and in interactive car simulators strong effects of marijuana on operational performance were found. In a study of Smiley et al. (1981) in an interactive driving simulator variability of velocity and lateral position increased during curve negotiation and while following cars and in windgusts. Variability of headway and lateral position while following cars also increased under marijuana. However, a larger headway was chosen during car-following under marijuana. In a study by Stein et al. (1983) with an interactive simulator, performance effects of marijuana were examined in a number of driving tasks such as car control during windgusts, curve following and lane changes. Although there were effects on steering performance, mean driving speed was lower under marijuana.
Several other studies have presented behavioural evidence suggesting that drivers may adapt their tactical behaviour to deteriorated operational performance by choosing a lower speed or by increasing headway in car-following. In an on-road study by Caswell (1977) drivers under marijuana drove more slowly. In an on-road study by Smiley et al. (1986) the effects of marijuana on several tasks such as car-following, curve following, open road driving, emergency decision making and obstacle avoidance were measured. Marijuana only had a few effects, but it significantly increased headway in the car-following task. Smiley (1986) concluded that all studies indicate that when the driver under marijuana has the possibility to choose a lower speed, there are no effects on lane position control while speed is reduced.Stein (1986) studied the effects of marijuana on driving behaviour in a number of driving tasks in a research driving simulator. A dose dependent effect of marijuana on speed was found; drivers decreased speed more with higher doses. In a task requiring the driver to compensate for random wind gusts, a strong effect of marijuana was found on mean speed and speed variability. Drivers were also required to control speed and steering during the negotiation of curves. Again, marijuana decreased speed. The speed reduction was also found in an obstacle avoidance task. No effects of marijuana on steering behaviour were found.
Robbe (1994) performed three on-road experiments in which the effect of marijuana on car driving was examined. In a study with driving on a restricted highway it was found that marijuana affected steering performance as indicated by an increased standard deviation of lateral position (SDLP). Subjects were instructed to maintain a constant speed of 90 km/h, or less if they felt incapable of driving safely at that speed. The greater the dose, the harder the subjects attempted to compensate as indicated by perceived effort and increased heart rate. Despite the instruction, there was a small reduction in mean speed under marijuana. Drivers rated the quality of their own driving performance lower with higher doses, suggesting that they were aware of the effects of marijuana.
In another experiment, Robbe (1994) had subjects drive on a highway with other traffic under the instruction to maintain a speed of 95 km/h. This also involved a car-following test in which subjects were instructed to maintain a 50 meter headway. A marijuana dose-dependent increase in SDLP was found and a decrease in speed under marijuana. Also, under marijuana headway increased although the increase was highest with the smallest dose. Reaction time to speed changes in the preceding vehicle increased under marijuana. However, reaction time was confounded with headway, such that RT increased with increased headway.
In a third experiment, Robbe (1994) examined the effects of marijuana in a city driving task. Driving performance was evaluated by trained observers (driving instructor). No effects of marijuana were found on driving performance. Under marijuana it took more time to complete the circuit, suggesting a lower speed, although this was not significant. Drivers under marijuana perceived their driving quality as poorer compared to placebo and perceived their effort as higher.
In conclusion, the studies of the effects of marijuana suggest that, firstly, it affects perceptual and psycho-motor skills, secondly, it affects performance on the operational level, and thirdly, it affects behaviour on the tactical level, especially when the task is self-paced. Evidence was presented that the drivers are aware of performance decrements under marijuana. It may be hypothesized that the perception of feedback of these performance decrements is a necessary prerequisite for such a compensation strategy. However, the nature of the perception of feedback, whether it is conscious or unconscious, is at present unclear. When the task is self-paced instead of prescribed by the experimenter (by instructing the subject to maintain a fixed speed), effects of marijuana on operational performance may be limited due to compensation for decreased skills: when drivers are allowed to choose their speed, effects of marijuana on steering behaviour are generally absent, while effects on steering behaviour are found when speed is prescribed by the experimenter. This compensation mechanism may explain why epidemiological studies have been unable to find a relation between marijuana and accident involvement.
The following literature was referred to:
- Caswell , S. (1977). Cannabis and alcohol: Effects on closed course driving behaviour. In: Johnson, L. (ed.). Seventh International Conference on Alcohol, Drugs, and Traffic Safety. Melbourne. Australia.
- Moskowitz, H. (1985). Marihuana and driving. Accident Analysis & Prevention, 17, 323-345.
- Robbe, H.W.J. (1994). Influence of marijuana on driving. Thesis. State University Limburg, Maastricht, The Netherlands.
- Smiley, A.M.; Moskowitz, H. and Zeitman, K. (1981). Driving simulator studies of marijuana alone and in combination with alcohol. Proceedings of the 25th conference of the American Association for Automotive Medicine, 107-116.
- Smiley, A.M. (1986). Marijuana: On-road and driving simulator studies. Alcohol, drugs and driving: Abstracts and reviews, 2, 135-154.
- Smiley, A.M.; Noy, I. and Tostowaryk, W. (1986). The effects of marijuana alone and in combination with alcohol on driving performance. In: Noordzij, P.C., Roszbach, R. (eds). Alcohol, drugs and traffic safety-T86. Elsevier Science Publishers BV, North-Holland, 203-206.
- Stein, A.C.; Allen, R.W.; Cook, M.L. and Karl, R.L. (1983). A simulator study of the combined effects of alcohol and marijuana on driving behaviour. Report submitted to the National Highway Traffic Safety Administration. Hawthorne, CA: Systems Technology Inc.
- Stein, A.C. (1986). A simulator study of the effects of alcohol and marihuana on driving behaviour. In: Noordzij, P.C., Roszbach, R. (eds).Alcohol, drugs and traffic safety-T86. Elsevier Science Publishers BV, North-Holland, 197-201.