Reducing cannabis-impaired driving: is there sufficient evidence for drug testing of drivers? : comment on "Developing limits for driving under cannabis", Addiction. 2007 Dec;102(12):1910-7.

There is increasing evidence that cannabis users who drive while intoxicated put themselves and others at increased risk of motor vehicle crashes. Cannabis produces dose-related cognitive and behavioural impairments in laboratory and simulator studies [1–3]; cannabis users in surveys are more likely to report being involved in accidents than drivers who do not use the drug (e.g. [4,5]), and cannabis is the illicit drug detected most often in drivers who have been killed in motor vehicle crashes (see [6] for a review). (Author/publisher).