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.

It’s difficult to know how accurate the roadside survey actually is. The NHTSA sets up designated data collection sites and uses road signs to tell drivers that the sites are coming up. Participation is completely voluntary.

According to measures of traffic on weekend nights, about 8 percent of drivers had alcohol in their system, with just less than one percent above the legal blood-alcohol limit. These findings represent an 80 percent decline in drunk driving (reported in the survey) since 1973, and a 30 percent drop since 2007.

In contrast, about 20 percent of drivers (on weekend nights) were found to have drugs in their system, which is up from 16.3 percent in 2007. It is unclear whether there was any differentiation between legal prescriptions and illegal street drugs. It is also unclear whether researchers tested for drug impairment or just the presence of drugs in the body. Researchers did say, however, that drivers found to have marijuana in their system has increased by about 50 percent since the previous survey.

Because recreational marijuana is now legal in Washington state (and medical marijuana is legal in many states), a rise in marijuana-impaired driving was somewhat expected. Washington has a DUI law regarding marijuana impairment, but many states have no statutes specifically addressing permissible levels.

Whether you are facing drunk driving or drugged driving charges, the stakes are high. As such, you should seek the help of an experienced DUI defense attorney.