Federal regulation limits interstate truck drivers to 10 hours of driving after an 8-hour off-duty period. The need for enforcing these limits is supported by research showing that long driving hours is a risk factor for tractor-trailer crashes. To estimate the percentage of hours of service violators among long-haul tractor-trailer drivers, truck drivers were interviewed at an inspection site in Spokane, Washington and later observed arriving at inspection sites in either Moorhead or Worthington, Minnesota (approximately 1,200 miles). The sample used for the calculation of violators consisted of truckers driving alone who reportedly did not plan to make an interim pickup or delivery stop prior to arrival in Minnesota. The percentages of drivers violating the hours of service rules by more than one hour at average trip speeds ranging from 35 mph to 65 mph are presented. Assuming that the drivers averaged 40 mph over the complete course of the trip segment, including stopped time, 90% were in violation by more than one hour. Assuming that they averaged 50 mph, 51% were in violation by more than one hour. These speed assumptions are based on findings in the current study that team drivers averaged 38 mph, fleet managers' reports of scheduling single drivers at trip speeds of 45 mph to 47 mph, and reports in the literature that loaded tractor-trailers average 41 mph over flat terrain. Although the true percentage cannot be determined without knowing actual trip speeds, the estimated range of violators at probable speeds of 40 mph and 50 mph points to a substantial problem.