Air Brake Practice Test 2025 – 400 Free Practice Questions to Pass the Exam

At 55 mph on a dry roadway, the lag time associated with air brakes is significant in determining the overall stop distance. The lag time refers to the delay between the moment the driver applies the brakes and when the brakes actually begin to engage due to the time it takes for the air to travel through the system and apply pressure to the braking mechanism.

At 55 mph, the vehicle travels approximately 1.47 feet per second for each mile per hour, which translates to about 80 feet per second. With an air brake lag time of approximately 0.5 seconds, the vehicle continues to travel during this delay. Multiplying the speed by the lag time, the distance added due to the lag time can be calculated as:

55 mph * 1.47 ft/sec = 80.85 ft/sec

80.85 ft/sec * 0.5 seconds = about 40.43 feet.

However, when rounding and considering factors such as driver response and actual conditions on the road, it can be approximated that the lag time contributes around 32 feet to the overall stopping distance. This figure indicates an essential factor for drivers operating vehicles equipped with air brakes to be aware of—anticipating the increase in stopping distance due to lag