Following the legalisation of marijuana for recreational use in Washington state in the United States the number of drivers involved in fatal crashes who tested positive for the substance has doubled.

However, a study by the AAA Foundation for Traffic Safety found only a correlation, not a causative link. Over the study period, traffic fatalities declined overall. The study also reflects some of the unknowns and difficulties that remain in consistently analysing marijuana's effects on drivers and creating the sort of legal framework and law enforcement methods used to combat drunken driving.

Washington Post