Michigan CDL - Air Brakes Practice Exam

At a speed of 55 mph, how much can brake lag add to the vehicle's total stopping distance?

1. 16 feet

2. 24 feet

3. 32 feet

4. 48 feet

The correct answer is: 32 feet

Brake lag is the time it takes for the driver to react and apply the brakes after noticing the need to stop, plus the distance the vehicle travels during that reaction time. At 55 mph, the perception and reaction time can result in a significant distance added to the total stopping distance. When driving at 55 mph, it is generally estimated that for every second of brake lag, a vehicle will travel approximately 45 to 50 feet—this is due to the speed at which the vehicle is moving. The additional distance caused by the brake lag is typically calculated using a common formula that considers the speed of the vehicle (in this case, 55 mph) and the time it takes to react. Given that brake lag can add approximately 32 feet to the stopping distance at 55 mph, this option accurately reflects the typical distance added under those conditions. It is essential for drivers to understand this concept as it highlights the importance of maintaining a safe following distance to allow for the necessary reaction time and stopping distance in emergency situations. This knowledge helps in making better driving decisions, especially in vehicles equipped with air brakes, which may have different braking characteristics compared to regular brakes.