A young woman smokes marijuana before the 10th annual Marijuana March in downtown Toronto May 3, 2008. REUTERS/Mark Blinch (CANADA)

Drivers who smoke marijuana within three hours of getting behind the wheel run almost double the risk of a serious crash.

A major review of accident data involving 50 000 drivers has revealed that those who have taken the class B drug are far more likely to be involved in a collision with another vehicle.

It comes amid growing concern about the influence of drugs on road safety, particularly among young drivers.

The latest research, published in the British Medical Journal, singled out drivers who tested positive for marijuana through blood tests, or who admitted using the drug before an accident.

It found that smoking marijuana resulted in a ‘near doubling of risk of a driver being involved in a collision resulting in serious injury or death’.

The researchers, from Dalhousie University in Canada, said: “The results also agree with recent data that point to the increasing presence of drugs other than alcohol (especially cannabis and depressants) in injured and fatally injured drivers.’

In the UK, drug-driving laws are to be tightened, with the possible introduction of roadside ‘drugalysers’, after a campaign by the family of 14-year-old Lillian Groves, who was run over and killed by a driver high on drugs in 2010.

Road safety minister Mike Penning said: ‘Drug-driving is a menace, which is why we are going to introduce a new drug-driving offence.’ - Daily Mail