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Marijuana or Cannabis and Driving: No Apparent Connection Between Cannabis Use and Automobile Crashes, According to the NHTSA


Dr. James C. Munch.– testified before Congress in 1937 that pot was bad, then got a job as pot-watcher for the government for 25 years (per a commenter on LA Times web site.)

A July 2017 report to Congress by the National Highway Traffic Safety Administration (NHTSA): “Marijuana-Impaired Driving: A Report to Congress”

“An interesting finding from this research is that after smoking marijuana, subjects in most of the simulator and instrumented vehicle studies on marijuana and driving typically drive slower, follow other
cars at greater distances, and take fewer risks than when sober (Stein, et al., 1983; Smiley, et al., 1981;  Smiley, et al., 1986; Casswell, 1977; Robbe and O’Hanlon, 1993). These effects appear to suggest that
the drivers are attempting to compensate for the subjective effects of using marijuana. In contrast, subjects dosed with alcohol typically drive faster, follow at closer distances, and take greater risks.”

“The existing epidemiological research (both culpability and case-control studies) have produced contradictory estimates of risk for marijuana use.
Some of these studies have suggested that marijuana use has minimal or no effect on the likelihood of crash involvement, while others have estimated a small increase in the risk of crash involvement.”

“NHTSA’s “Crash Risk” Study
This case control crash risk study is the first large-scale study in the United States to include drugs other
than alcohol. It was designed to estimate the risk associated with alcohol- and drug-positive driving.
Virginia Beach, Virginia” was the site used and 3,000 crashes were studied.  A case-control method was used, and crash drivers were compared with non-crash drivers on the same roadway, at the same time of day and week.  The results were disappointing: after controlling for every variable, including age and sex, and especially alcohol use, no relation between THC-positivity and crash risk was found.  Unfortunately, there is no good test available to determine when the last time someone had smoked marijuana had occurred; only THC in the blood or saliva could be easily measured, and these don’t directly correspond to time of consumption.  THC remains in the blood for days after consumption.

“The drug most frequently detected in the oral fluid and blood of drivers was THC, detected in 7.6 percent (n = 234) of the crash-involved drivers and 6.1 percent (n = 379) of the control drivers. ”

“When the odds ratios were adjusted for demographic variable of age, gender, and race/ethnicity the
significant increased risk of crash involvement associated with THC disappeared. The adjusted odds
ratio for THC positive drivers was 1.05 (95% Confidence Limit of 0.86 – 1.27). This adjusted odds ratio
was not statistically significant.
A final adjustment was made for the presence of alcohol. When both demographic variables and the
presence of alcohol were taken into account, the odds ratio for THC declined further to 1.00
(95% Confidence Limit of 0.83 – 1.22). This means there was no increased risk of crash involvement
found over alcohol or drug free drivers.
As was described above, there was no difference in crash risk for marijuana-positive drivers who were
also positive for alcohol than for marijuana-positive drivers with no alcohol, beyond the risk attributable
to alcohol. Further analyses examined the potential interaction between drug use and breath alcohol
concentration. No statistically significant interaction effect on crash risk was found between for
THC positive drivers and BrAC level.
More information on the methodology of this study is available in a Research Note (Compton and
Berning, 2015 which can be downloaded at:

The defect in this study and all others is that there is no known reliable method (other than self-report, which is of questionable reliability) for determining when the last time the pot was consumed.   This makes all the difference, as the studies in the laboratory all show at least mild impairment of driving-related skills for two to three hours after consumption.  Whether increased caution compensates for these impairments is not known but should be considered.  Therefore, the only conclusion that we can reach is that people who smoke pot at least occasionally show no greater risk for crashing while driving than completely sober individuals.  This is comparable to what is known of drinking: people who drink occasionally are no worse drivers than people who don’t drink at all.  However, we know from these studies that people who are impaired by drinking are terrible drivers.  No such information is available in relation to the acute consumption of cannabis within three hours before or even while driving.

(photo courtesy of

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