Updating the Grade Severity Rating System (GSRS) for Wyoming Mountain Passes: A Description of Tests and Results

Truck crashes on Wyoming mountain passes due to brake heating has been a long-standing issue due to the steep downgrades that characterize some routes in the state. The grade severity rating system (GSRS) developed by the Federal Highway Administration (FHWA) to recommend maximum safe speeds has been identified as a viable countermeasure to reducing the incidence of downgrade truck crashes. However, several decades have passed since the GSRS was developed. In the decades since its development, truck features have undergone radical changes in terms of design. The streamlined design of tractors and trailers, use of drag reduction devices, changes in engine features, and adoption of radial tires have led to a reduction in the non-brake forces that retard motion. Truck brakes have also changed along with retarder characteristics. This has meant that maximum safe speeds recommended by the GSRS have been conservative. This article discusses tests and results involved in updating the GSRS. Maximum safe speeds from the updated GSRS model was compared to the previous FHWA model and was found to be higher while allowing for faster descent. The effect of the GSRS on retarder use was also evaluated using the updated model. The updated GSRS model should increase driver confidence in recommended speeds while improving Wyoming mountain pass safety.