Illinois CDL Practice Exam 2025 - Free CDL Practice Questions and Study Guide

Brake lag is the time it takes for the air pressure to build up and activate the brakes after the driver has applied the foot pedal. This delay can significantly impact the total stopping distance of a vehicle equipped with air brakes because it introduces a gap between the driver’s action and the moment the brakes actually engage.

When brake lag is factored in, the total stopping distance includes both the perception-reaction distance (the distance traveled while the driver recognizes the need to stop) and the braking distance (how far the vehicle travels while it comes to a complete stop). Since brake lag increases the time it takes for the brakes to respond, this means that the vehicle continues to travel forward for a longer distance before the brakes start to act, thus extending the overall stopping distance.

Factors such as the vehicle's speed, the condition of the brakes, and the surface on which the vehicle is traveling can further impact stopping distance, but the primary reason for the increased stopping distance due to brake lag is that there is a measurable delay in brake activation. Therefore, acknowledging how brake lag contributes to a longer stopping distance is crucial for maintaining safety on the road.