While drunk driving is known to be a common problem, it is difficult to measure or accurately predict how prevalent the behavior is. Advocacy groups often pull statistics on alcohol-related crashes and drunk-driving arrest rates, but these data do not definitively reveal how many motorists are driving under the influence on U.S. roads and highways.
Government regulators like the National Highway Traffic Safety Administration also try to gather drunk driving statistics, but they do so in a unique way. Every eight years or so, the NHTSA conducts its anonymous "roadside survey." According to the most recent survey's results, drunk driving rates appear to be declining, while drugged driving is up - especially drivers with marijuana in their system.