Drug War Distortions: Cannabis And Driving

"Each year, for example, marijuana use is linked to tens of thousands of serious traffic accidents."
Source: John P. Walters, "The Myth of 'Harmless' Marijuana," Washington Post, May 1, 2002, p. A25.

False.

A number of studies on the effects of cannabis on driving have been conducted over the past few decades, in the US and internationally. Though driving under the influence of any controlled substance is a matter of concern, there is no evidence that cannabis use alone is a significant cause of motor vehicle accidents. The European Monitoring Centre on Drugs and Drug Addiction noted in its 1999 literature review on drugged driving that "Field studies have demonstrated that cannabis is one of the most prevalent drugs discovered in fluid samples taken from drivers. However, assessment of the causal role of cannabis in accident occurrence is complicated by the fact that alcohol is also present in the majority of samples."
Source: European Monitoring Centre for Drugs and Drug Addiction. Literature Review on the Relation between Drug Use, Impaired Driving and Traffic Accidents. (CT.97.EP.14) Lisbon: EMCDDA, February 1999. p. viii.

This distortion is closely related to another distortion regarding marijuana and emergency room visits. See Distortion 6: Emergency Room Visits for more information, but generally, mentions of cannabis in emergency department visits does not mean that the drug was the *cause* of the visit. The federal Drug Abuse Warning Network's report for 2000 says:

"Marijuana/hashish mentions related to all motives were stable from 1999 to 2000 (Table 26). ED contacts due to chronic effects increased 25 percent (from 6,891 to 8,621), and contacts due to patients seeking detoxification increased 18 percent (from 11,908 to 14,110). However, 2 important caveats must be kept in mind. First, the drug use motive and reason for ED contact were frequently unknown or reported as ‘other’ (24% and 23% of mentions, respectively). Second, drug use motive and reason for ED contact pertain to the episode, not a particular drug. Since marijuana/hashish is frequently reported in combination with other drugs, the reason for the ED contact may be more relevant to the other drug(s) involved in the episode."
Source: "Year-End 2000 Emergency Department Data from the Drug Abuse Warning Network," Office of Applied Studies, Substance Abuse and Mental Health Services Administration, US Dept. of Health and Human Services, July 2001. p. 21)

Probably the best study of cannabis and driving was conducted for the Road Safety Division of the UK's Department of the Environment, Transport and the Regions, by the Transport Research Laboratory, Ltd., in 2000. They concluded that, "Overall, it is possible to conclude that cannabis has a measurable effect on psycho-motor performance, particularly tracking ability. Its effect on higher cognitive functions, for example divided attention tasks associated with driving, appear not to be as critical. Drivers under the influence of cannabis seem aware that they are impaired, and attempt to compensate for this impairment by reducing the difficulty of the driving task, for example by driving more slowly."
Source: Sexton, BF, RJ Tunbridge, N Brooke-Carter, et al., "The Influence of Cannabis on Driving," Prepared for Road Safety Division, Department of the Environment, Transport and the Regions, UK, by Transport Research Laboratory, Ltd., TRL Report 477, 2000, p. 4.

The report further notes that alcohol has a more severe, negative effect on driving than does cannabis: "In terms of road safety, it cannot be concluded that driving under the influence is not a hazard, as the effects on various aspects of driver performance are unpredictable. In comparison with alcohol however, the severe effects of alcohol on the higher cognitive processes of driving are likely to make this more of a hazard, particularly at higher blood alcohol levels."
"The Influence of Cannabis on Driving," p. 29.

Importantly, the TRL report concludes that it is possible for law enforcement to determine whether a person is impaired by cannabis, though they do recommend that more work be done to refine the techniques: "On the basis of these observations, the general medical examination and standardized impairment testing applied by the police surgeons were judged to be effective in determining both impairment and establishing condition due to a drug. Preliminary conclusions were drawn by the police surgeons on the number and combination of impairment test failures which would allow a conclusion that the driver was 'impaired'. Further refinement and calibration of these techniques in the field, for use by both police officers and police surgeons, is however desirable and is planned."
"The Influence of Cannabis on Driving," p. 29.

Distortion 12: Cannabis and Driving