A study indicates that those who smoke a moderate amount of marijuana are only barely more dangerous than completely sober drivers.

While the researchers do not call for cannabis to be legalised, they do want people to acknowledge that the effects of the drug on driving have been overstated.

But the research runs contrary to earlier research which, road safety campaigners say, proves drug driving is just as dangerous as drink driving.

Comparative study

The research was led by Alison Smiley and conducted at the University of Toronto.

She compared studies that looked into how serious the impairment was from marijuana compared with alcohol.

Drugs are involved in 16% of crashes

She said: "Both substances impair performance.

"However, the more cautious behaviour of subjects who received marijuana decreases the drug's impact on performance.

"Their behaviour is more appropriate to their impairment, whereas subjects who received alcohol tend to drive in a more risky manner."

Ms Smiley said that earlier studies on the subject had given people "fairly hefty doses" of cannabis and then put the drugged volunteers behind the wheel right afterwards.

She said such studies may have exaggerated the effects of the drug.

"There's an assumption that, because marijuana is illegal, it must increase the risk of an accident. We should try to just stick to the facts," she said.

Her findings were published in Health Effects of Cannabis, which is published by Toronto's Centre for Addiction and Mental Health.

Call for action

Last year, medical experts consulted by the Automobile Association demanded the government take urgent action to cut the death toll caused by motorists using illegal drugs.

They said drugs were involved in 16% of road deaths - with cannabis identified in two-thirds of fatal accidents.

The doctors said most drugged drivers were unaware they were breaking the law and police lacked the resources to act.

However, they added that, while it was easy enough to test whether someone had taken drugs, it was much more difficult to prove that drugs impaired driving.