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Cannabis & Driving
by Erowid
While it is widely accepted that cannabis use can alter motor skills and interfere with task attention, with higher doses causing strong effects, research has shown that cannabis use is less likely to dangerously impair driving abilities than alcohol at similar levels of intoxication. Cannabis intoxication often makes smokers more aware of their impairment, causing them to slow down and become more cautious while also worsening reaction time and attention. Cannabis users often report that driving speeds are experientially 'faster' than normal: driving a given speed feels faster and more dangerous than the same speed does while sober.

Dozens of studies have looked at this issue and most have found that cannabis smoking does degrade driving performance moderately. There is some contradictory evidence about whether cannabis in combination with alcohol causes worse impairment than alcohol alone, but so far the data heavily favors the view that the combination substantially increases risks over either alone. Reliable scientic research, as of 2015, does not provide a clear answer to how much accident risks increase with moderate levels of cannabis intoxication, but only confirms that the risks of cannabis-alone impairment are lower than those of alcohol-alone impairment. There is even less reliable information about how lingering low levels of cannabinoids in the blood might increase (or reduce) risk of accident. The question of whether smoking cannabis 1-7 days prior impacts driving performance has not been sufficiently addressed. The following are a collection of summaries & papers which look at the issue of cannabis & driving performance.

Misc Reports
Is It Safe to Drive While Stoned? Cannabis and Driving: An Erowid Science Review - by Andrew Sewell, MD, Feb 4, 2010
Effects of marijuana (with and without alcohol) on driving performance - NIDA (2015)
Executive Summary of Driving Impairment Effects of Alcohol & Cannabis (1994)
The influence of cannabis on driving, TRL, Britain (PDF - 1.5MB)
UK Lords Report On Cannabis & Driving

News & Media
Alcohol impairs driving more than marijuana - New Scientist March 2002

"A single glass of wine will impair your driving more than smoking a joint. And under certain test conditions, the complex way alcohol and cannabis combine to affect driving behaviour suggests that someone who has taken both may drive less recklessly than a person who is simply drunk."
New Scientist March 2002
Journal References
Testing Reckless Drivers for Cocaine and Marijuana

Marijuana Myth: "Marijuana Is A Major Cause Of Highway Accidents"

US National Highway Traffic Safety Administration Reports
Marijuana And Actual Driving Performance - by Robbe, NHTSA, 1993
Marijuana Use And Driving, by Robbe 1994
Marijuana And Actual Driving Performance - by Robbe, NHTSA, 1999
Marijuana & Alcohol Combined Increase Impairment - NHTSA 1999
"Drivers under the influence of marijuana retain insight in their performance and will compensate where they can, for example, by slowing down or increasing effort. As a consequence, THC's adverse effects on driving performance appear relatively small."
Robbe, NHTSA 1993


NORML: Driving and Marijuana
Cannabis Campaigner's Guide to Cannabis & Driving
Schaffer Library References on Drugs and Driving
Cannabis And Road Safety: An Outline Of The Research Studies To Examine The Effects Of Cannabis On Driving Skills And On Actual Driving Performance

SAMHSA: Steer Your Teen Away from Marijuana

"Professor Hall considers cannabis's contribution to danger on the roads to be very small; in his view the major effect of cannabis use on driving may be in amplifying the impairments caused by alcohol."
UK Lord's Report, 1998