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Heavy Vehicle Brake Research
This ongoing research program is aimed at improving the braking performance of heavy vehicles by supporting the development and evaluation of advanced braking systems. Recent major research areas include air disc brakes, next-generation ABS, electronically controlled braking systems (ECBS), and technologies enabled by ECBS such as stability control, collision mitigation, and adaptive cruise control (ACC).

ECBS builds on existing ABS technology in that the air signal traditionally used by ABS to control the activation of the vehicle foundation brakes is replaced by an electronic signal. This reduces the time needed to activate the brakes, resulting in faster vehicle response time and, potentially, a shorter stopping distance. The brake torque generated at each wheel is still provided by air pressure delivered to the brake chamber, but the air pressure is applied and controlled electronically. FMVSS 121 (air brakes) currently requires brake control redundancy in the form of a dual pneumatic brake system. This means ECBS must be overlaid on an existing air brake system, resulting in two pneumatic control circuits and one electronic control circuit (2P/1E). Since this is not cost effective, research is currently underway to define future regulations for 1P/1E and 2E systems. Other important goals of the research are to evaluate the performance, reliability, maintenance costs, and user acceptance of ECBS.

Due to technological advances in medium and heavy duty vehicle brakes, NHTSA believes stopping distances for these vehicles can be decreased substantially. Agency research indicates medium and heavy duty brake system performance improvements can be accomplished with upgraded foundation brake systems using various axle configurations with disc brakes and/or higher-output s-cam brakes.

Recent research has focused on an upgrade to FMVSS 121 that will require a 30-percent reduction in the stopping distance of heavy truck tractors. Future rulemaking activity may focus on all vehicles covered by FMVSS 121, and also FMVSS 105 (hydraulic & electric brakes). Upgraded versions of these regulations might include:
   - Shorter stopping distances requirements for buses, single-unit trucks, and empty truck-tractors on a high-coefficient-of-friction surface.
   - Increased brake retardation force or stopping distance requirements for air-braked semi-trailers, trailers, and converter dollies towed by truck tractors.

U.S. Department of Transportation National Highway Traffic Safety Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
1-800-424-9153 (TTY)