Expressway Gradient Threshold Analysis Based on Driving Safety

Abstract Brake failure and poor braking are significant causes of traffic accidents. To establish the longitudinal slope gradient design criteria ensuring expressway operational safety, this study investigates the gradient thresholds under two braking modes of engine‐assisted and exhaust‐assisted at four truck gears through theoretical calculations. Real vehicle tests on the Ya'an‐Xichang Expressway were conducted to capture driver heart rate characteristics. The threshold that ensures driver psychological safety was determined based on heart rate increments. The findings indicate that as gradient increases, the length of longitudinal gradients decreases until a critical threshold, beyond which it remains unlimited. The gradient threshold decreases linearly with increasing truck braking gears. Specifically, a 2.80% gradient threshold ensures braking safety for trucks descending in 12th gear. During extended longitudinal gradient driving on highways, heart rate increments follow a normal distribution. A gradient threshold of 2.96% ensures driver psychological safety. A 2.80% gradient threshold on highways meets both vehicle and driver safety requirements.