Recent reports have surfaced making claims that in places where marijuana has been legalized, that car crashes and deaths involving marijuana have increased, but the data has been skewed in favor of opponents and does not tell the whole truth. Some articles are claiming that more accidents are occuring with drivers testing positive for marijuana in Colorado, but that doesn't mean drivers were high or impaired at all during the accident as marijuana can stay in your body days after the high has worn off, sometimes weeks. In fact, a well rounded article would also include that driving fatalities in Colorado decreased in 2014 compared to 2013 - after recreational marijuana became legal in the state. A true judge of impairment will come from a sobriety test administered by an officer, not the amount of THC stored in someone's body fat.
That of course is why the proponents of legalization here point out that detectable residual metabolites has nothing necessarily to do with driving impairment, and therefore a far fairer and more meaningful system is a field sobriety test, as commonly used in testing for alcohol impairment.