Impact of Fuel Conservation Measures on Safe Truck Downgrade Speeds

Runaway truck accidents are primarily due to brake fade and failure resulting from excessive temperature of the truck system brake components generated while descending severe downgrades. A grade severity rating system (GSRS) was developed in 1982 that determined the maximum safe downgrade speeds for various categories of truck gross weights. Heavy trucks, without retarders, that adhere to the maximum recommended descent speed have sufficient braking capacity to prevent a runaway condition. Since the development of the GSRS model, the use of radial truck tires and the manufacture of streamlined truck tractors have increased substantially. These fuel conservation measures reduce the nonbraking forces and place a greater load on the truck system brakes when it is necessary to maintain truck speed on downgrades. The impact of fuel conservation measures on truck system brake temperature is the equivalent of descending a downgrade which is 0.34% steeper than actual conditions. In addition, fuel conservation measures lower the threshold of truck weight susceptible to excessively high brake temperatures, and subsequent runaway conditions, from 45,000 to 35,000 lb.