This study evaluated a device that prevents drivers from shifting vehicles into gear for up to 8 s unless seat belts are buckled. Participants were 101 commercial drivers who operated vans, pickups, or other light trucks from the U.S. and Canada. The driver could escape or avoid the delay by fastening his or her seat belt before shifting out of park. Unbelted participants experienced either a constant delay (8 s) or a variable delay (M = 8 s). A 16-s delay was introduced for those U.S. drivers who did not show significant improvement. Seat belt use increased from 48% to 67% (a 40% increase) for U.S. drivers and from 54% to 74% (a 37% increase) for Canadian drivers. The fixed delay was more effective for U.S. drivers than the variable delay, but there was no difference between these two delay schedules for Canadian drivers. After the driver fastened his or her seat belt, it tended to remain fastened for the duration of the trip. (Contains 3 tables and 2 figures.)