Smoking pot before driving doubles the risk of a serious or fatal car crash, according to a new Canadian study. The research by professor Mark Asbridge of Dalhousie University also found that the number of Canadian driving under the influence of marijuana is growing; in some places it is more common than drinking in driving. Previous research looking into the link between marijuana consumption and the risk of car accidents had yielded different results, so the Dalhousie team pooled data from nine studies involving 49,411 drivers from Australia, New Zealand, the United States, France and the Netherlands. The researchers discarded all cases where alcohol or other substances had been found in the blood of drivers who either tested positive for marijuana or reported to have been smoking within three hours of the crash. They found drivers who had smoked pot had a 92 per cent higher risk of being involved in an accident causing serious injury or death to themselves or other people.