UK Transport Research Laboratory in August 2000, which found that pot-smoking has a minimal if not beneficial effect on driving performance.
The government-funded study was launched under pressure from anti-drug and driving groups, and was an embarassment to the British Ministers who had expected it to support their anti-stoned-driving campaigns.
The British study confirmed the results of a wide variety of research into stoned driving from around the world:
A 1983 study by the US National Highway Transportation Safety Administration (NHTSA) used stoned drivers on simulators, and concluded that the only statistically significant effect associated with marijuana use was slower driving.
A comprehensive 1992 study by the NHTSA found that marijuana is rarely involved in driving accidents, except when combined with alcohol. It concluded that "the THC-only drivers had an [accident] responsibility rate below that of the drug free driversS While the difference was not statistically significant, there was no indication that cannabis by itself was a cause of fatal crashes." This study was buried for six years and not released until 1998.
Another NHTSA study performed in 1993 dosed Dutch drivers with THC and tested them on real Dutch roads. It concluded that "THC's adverse effects on driving performance appear relatively small."
A massive 1998 study by the University of Adelaide and Transport South Australia analyzed blood samples from 2,500 accidents, and found that drivers with cannabis in their system were actually slightly less likely to cause accidents than those without.
A University of Toronto study released in March 1999 found that moderate pot users typically refrained from passing cars and drove at a more consistent speed than non-users.
An important consideration when considering the effects of cannabis and driving is whether the smoker is an experienced user. Novice tokers typically experience more difficulty driving than regular users.
The British study also found that tiredness caused 10% of all fatal accidents, compared with 6% for alcohol.