The primary purpose of the study will be to determine whether marijuana decriminalization in California is associated with an increase in the prevalence of marijuana-positive driving.

A growing number of states are decriminalizing and, more recently, legalizing marijuana possession; yet few if any studies have examined whether these laws contribute to an increase in marijuana use among drivers or impact traffic safety. California passed a marijuana decriminalization law (Senate Bill 1449) in 2010. Given the current dearth of information on marijuana-related laws and driving, the study will provide critical information on whether the liberalization of marijuana laws has any significant effect on driving under the influence of marijuana and marijuana-related crashes.