This report presents a statistical analysis of crash data in order to determine the effectiveness of antilock brakes in tractor-trailer combination vehicles. Federal Motor Vehicle Safety Standard No. 121, Air Brake Systems, mandates antilock braking systems on virtually all new air-braked vehicles with a GVWR of 10,000 pounds or greater. ABS is required on tractors manufactured on or after March 1, 1997, and air-braked semitrailers and single-unit trucks manufactured on or after March 1, 1998..