In 1999, National Highway Traffic Safety Admini-stration (NHTSA) researchers theorized that substan-tial improvements could be made in the braking per-formance of medium and heavy trucks. Therefore, NHTSA initiated a multi-year research program to learn what improvements in stopping performance could be achieved using advanced, but currently available, brake technology for medium and heavy trucks.

Truck tractors were the first type of heavy truck stud-ied. Tractor testing results, including dry stopping distance, wet brake-in-curve stability evaluations, and wet split coefficient of friction stopping distances are presented. Testing results showed that a 30 percent reduction in maximum permissible dry stopping dis-tances is possible for U.S. truck tractors, with no deg-radation in other performance areas. Objective measurements of brake torque, measured on NHTSA’s inertial brake dynamometer at speeds up to 112.7 kph, are presented. Vehicle dynamics simula-tion results were used to understand effects that higher-torque brakes might have on jackknife stabil-ity during braking of tractor-semitrailer rigs.

Changing tractors to have all air disc brakes make braking performance improvements attainable with incremental costs that are outweighed by the ex-pected benefits. Unforeseen improvements include a nominal 5 to 8 percent improvement in stopping dis-tance during ABS-controlled stops on wet pavement, a result of significantly lower brake hysteresis with air disc brakes. Hybrid brake configurations, utiliz-ing larger, more powerful S-cam drum brakes or air disc brakes on the steer axle only, are also shown to provide significant performance improvements over current foundation brakes. Based on this research, NHTSA has proposed revising FMVSS 121;​ shorten-ing the maximum permitted stopping distance for truck tractors by 20 to 30 percent.

The paper concludes by briefly discussing NHTSA’s research to improve the stopping performance of me-dium and heavy straight trucks.