Drivers who smoke cannabis three hours before driving are twice as likely to cause a car crash as those not under the influence of alcohol or drugs, unveils a Canadian study.

Research team from Dalhousie University, Nova Scotia, Canada, which reviewed nine studies of more than 49,000 people involved in accidents on public roads, found that drivers who had consumed marijuana within three hours of driving were twice as likely to cause a fatal collision.

The study also found that the risk of collision was higher if the driver was aged below 35, but failed to find how much of cannabis is needed to worsen the risk of a crash. The research published in the British Medical Journal has raised the concern about the influence of drugs on road safety, as the illicit substance is used by millions of Britons aged between 16 and 24 years.

After a campaign by the family of Lillian Groves, a 14 year old, who was killed by a driver high on drugs in 2010, the UK drug-driving laws are to be tightened and roadside 'drugalysers' are to be introduced.

The study refers to the roadside survey of 537 drivers in Scotland, which found that 15% of participants admitted to having smoked marijuana within 12 hours of operating a vehicle.

The European Monitoring Centre for Drugs and Drug Addiction in 2008 found that in the roadside surveys in the United Kingdom, Denmark, the Netherlands, Norway, the United States, and Australia, between 0.3% and 7.4% of drivers were tested positive for cannabis.

In spite of the increased risk of crash posed by cannabis, most car drivers consume alcohol before driving, concluded the researchers.