Car crash risk doubles with cannabis use
By Liz Lockhart
The chance of having a vehicle accident is nearly twice as likely when the driver has consumed cannabis within three hours of driving, 
according to a paper published recently. This not only applies to car accidents but also lorries, motor bikes, vans, sports utility vehicles and buses.
The paper authors are from Dalhousie University, Canada. They reviewed nine studies with a total sample of 49,411 people to determine if there was an increased risk of motor vehicle collision when cannabis had been consumed.
This is the first review to review observational studies concerned with the risk of vehicle accident after cannabis use. Previous studies have not been able to separate the effects of alcohol use and other substances from the use of cannabis. This has caused of a lack of agreement on the subject.
With cannabis being the most widely used illicit substance in the world and with recent statistics showing a significant increase in use globally, the rates of driving under the influence has also increased. In 2007 a survey was carried out in Scotland which showed that 15% of drivers tested aged 17-39 admitted to having used cannabis within 12 hours of driving. A total of 537 drivers were tested in this survey.
In this study, all motor vehicle collisions occurred on a public road and involved one or more moving vehicles. The results were obtained either through blood samples or direct self-reporting and show that if cannabis is consumed before driving a vehicle, the risk of an accident is nearly doubled. Drivers aged 35 or younger are also found to be at a substantially higher risk.
The authors of this paper suggest that the consumption of cannabis impairs motor tasks important to safe driving, increasing the chance of an a collision. In conclusion they say that future reviews should assess less severe collisions from a general driving population.
