Since 1973, the U.S. Department of Transportation has conducted five national surveys concerning impaired driving. The survey, called the National Roadside Survey of Alcohol and Drugged Driving, attempts to provide the best estimate of rates of impaired driving and is considered an important way to monitor driving safety. That said, the survey is not universally accepted, particularly because the methods used to gather the information sometimes push the limits legally speaking.
The survey takes place in 60 different cities nationwide, with motorists being randomly selected for questioning about their drinking and driving habits and asked for a roadside breath test. Motorists are offered money for saliva and blood samples and for answering more questions. The survey is often done by private contractors working for the government, but in some cases uniformed police officers conduct the survey.