Since recreational use of marijuana was legalized in Washington state at the end of 2012, fatal motor vehicle crashes involving drivers under the influence have more than doubled. The rate of fatal car crashes associated with marijuana use rose from 49 (8 percent) in 2013 to 106 (17 percent) in 2014, according to the nonprofit AAA Foundation for Traffic Safety.
Some states have set legal driving limits for marijuana, based on blood levels of the psychoactive ingredient THC (similar in concept to a blood alcohol limit for driving). But there’s much debate about what a safe limit (if any) is, since marijuana affects users differently and since impairment levels do not necessarily correspond with blood THC levels.
Marijuana is legal for medical or recreational use in 25 states and the District of Columbia, with several more states considering legislation.
Also see Marijuana: Hazy Health Risks.