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Cannabis Research - Safety - Driving

U.S. Department of Transportation
National Traffic Safety Administration
DOT HS 808 939

Marijuana, Alcohol and Actual Driving Performance

July 1999

The magnitudes of impairment observed after these doses of THC alone were not especially large in historical comparison to those of other drugs and alcohol present in BACs above 0.08 g/dl. However, they do imply a loss of driving ability that could be serious in exceptional traffic situations. The combination of THC with alcohol sufficient for attaining a BAC of about 0.04 g/dl has very severe effects on driving performance. Subjects showing those effects drove in a manner one would expect for drivers operating with BACs above the per se definition of intoxication in some states, i.e., 0.08 g/dl. P.6

Surveys conducted in widely separated localities have generally revealed the presence of THC in between 4 and 12% of drivers who sustained injury or death in crashes (Cimbura et al, 1980,1982; Terhune, 1982; Chesher & Starmer, 1983; Donelson et al, 1985; Garriott et al, 1986; Daldrup et al, 1987; McClean et al, 1987).

The latest and largest of the postmortem surveys (Terhune et al, 1992) came closest to discriminating among the separate and combined effects of THC and alcohol on crash risk. It involved a sample of 1,882 fatally.injured drivers from seven widely separated American States during 1990.91. drivers comprised 42.1% of the sample, and those showing the presence of alcohol, 51.5%. THC was found in only 4.3% and among them, three.quarters tested positive for alcohol as well

The investigators undertook two different analyses for inferring causal relationships between these drugs and crashes. First, they compared responsibility rates of subgroups using each drug separately and in combination with that for the group. Responsibility in this case was assigned by trained encoders using a standardized procedure for evaluating police reports. drivers were held responsible for 67.7% of their crashes. The responsibility rates for drivers showing only the presence of alcohol depended upon their BACs. For those with BACs below 0.10 g/dl it was 75.8%, and for those at or above that level, 93.9%. Drivers showing only the presence of THC were 57.9% responsible; i.e., less often than drivers, albeit not significantly. However, the group showing the combined presence of THC and alcohol in any concentration at all, were held responsible for 94.6% of their crashes. This rate differed significantly from the drivers' though not from the group's with the highest BACs.

The second analysis was for calculating the relative crash risk (RR) of each subgroup whose number was sufficient for providing a reliable estimate. "Non.responsible" drivers in the sample were defined as the control group, following the assumption that they were representative of the general driving population with respect to drug and alcohol prevalence. The investigators were aware that their definition of the control group is unconventional and urged caution in interpreting the results. But as they said, those results were certainly "suggestive." Relative to drivers whose M was defined as 1.0, those showing BAC<O. 10 g/dl operated with a RR=1.2. Drivers operating with BAC>= 0 10 g/dl did so with RR=6.5. Strikingly, drivers operating with THC and any BAC had a RR=11.9. Unfortunately, only 19 fatally injured drivers were found with only THC in their blood, too few for estimating the RR. P.7-8

No dose significantly affected this group's car following performance but all elevated SDLP in the Road Tracking Test. Relative to the placebo level, mean SDLP rose by 1.1, 1.8 and 2.9 cm in rough proportion to the THC dose. Though all significant, these changes were very modest in comparison to those produced by alcohol and a number of medicinal drugs in previous investigations. Alcohol was used as the agent for calibrating the standard test (Louwerens et al, 1987). Social drinkers performed it on five separate occasions while their BACs were controlled in equal steps between 0.00 and 0.15 g/dl. The drinkers' mean SDLP rose exponentially with BAC (R=0.99) and an empirical equation was derived for describing the relationship. The equation has been subsequently used for describing drugs' effects on SDLP in terms of respective BAC equivalencies. When we applied it for describing THC's effects in the earlier study, the maximum mean change after 300 ~g/kg was found to be the equivalent of BAC=0.08 g/dl. THC's effects were also far less than those of some commonly used medicinal drugs. For example, after a week of receiving diazepam 5 me (Valium.), thrice daily, and lorazepam 2 me (Ativan.), twice daily, different groups of clinically anxious patients drove with mean SDLPs that were respectively 7 and 10 cm higher than baseline (Van Laar et al, 1992; O'Hanlon et al, 1995). Thus, we concluded that THC taken alone in doses preferred by its users does not seriously affect driving performance. P.22-23

Alcohol present in blood concentrations around the legal limit (0.10 g/dl) in most American States is more impairing than anything subjects have shown after THC alone in our studies. As mentioned, medicinal drugs have had worse effects on psychiatric patients' driving performance in other studies employing the same test procedures. P.26

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