LONDON — People who use marijuana before driving are nearly twice as likely to cause a car crash as those not under the influence of alcohol or drugs, according to a Canadian analysis of previous studies.

Experts at Dalhousie University in Halifax reviewed nine studies of more than 49,000 people involved in accidents on public roads involving one or more motor vehicles, including cars, trucks, buses and motorcycles. Marijuana use was confirmed by blood tests or self-reporting.

Researchers found drivers who had used marijuana within three hours of beginning to drive had nearly double the risk of causing a collision, especially those that were fatal.

Marijuana is the most widely used illegal drug worldwide and rates of its use in drivers are increasing. A 2007 study in Scotland found 15 per cent of 537 drivers aged 17 to 39 had used marijuana within 12 hours.

Some experts said education campaigns about the dangers of doing drugs before driving wouldn't work.

People "will also need to be persuaded that they are at risk of their cannabis use being detected," wrote Wayne Hall of the University of Queensland in an accompanying editorial.

Hall said more research was needed to prove whether roadside drug testing, as introduced in parts of Australia and the U.S., actually prevents more drug-related car accidents.

The study was published Friday in the journal, BMJ.