Long driving hours have been shown to be a risk factor for tractortrailer crashes. To estimate the prevalence of drivers at risk, truckers were interviewed at an inspection site in Spokane, Washington and later observed arriving at inspection sites in either Moorhead or Worthington, Minnesota (1,200 miles). Hours of service violators were estimated using the sample of truckers driving alone who reportedly did not plan to make an interim pickup or delivery stop prior to arrival in Minnesota. Assuming that the drivers averaged 40 mph over the complete course of the trip segment, including stopped time, 91 percent were in violation of the hours of service rules;​ 90 percent were in violation by more than one hour. Assuming that they averaged 50 mph, 58 percent were in violation;​ 51 percent were in violation by more than one hour.