A commercial vehicle with air brakes is traveling at 45 mph on a dry, level road. The driver needs to perform an emergency stop. What is the approximate total stopping distance for this vehicle?
- AApproximately 216 feet
- BApproximately 120 feet
- CApproximately 450 feet
- DApproximately 300 feet
Show answer & explanationAnswer & explanation
Correct answer: A. Approximately 216 feet
The total stopping distance for a commercial vehicle with air brakes is comprised of perception distance, reaction distance, and brake lag distance, plus actual braking distance. While exact figures vary, a common rule of thumb for 45 mph is roughly 216 feet. This is derived from a perception distance of about 33 feet (0.75 seconds), a reaction distance of about 50 feet (1.5 seconds), a brake lag distance of about 45 feet (1 second), and a braking distance of approximately 88 feet. (33 + 50 + 45 + 88 = 216 feet)
Why the other options are wrong
- B. This is significantly less than the typical total stopping distance for a commercial vehicle at this speed.
- C. This is a significant overestimate, likely representing much higher speeds or adverse conditions.
- D. This is an overestimate for 45 mph, closer to what might be expected at higher speeds like 55 mph.
Total Stopping Distance
The total distance a vehicle travels from the moment a driver perceives a hazard until the vehicle comes to a complete stop.
- Comprises Perception Distance, Reaction Distance, Brake Lag Distance, and Braking Distance.
- Increases significantly with speed and vehicle weight.
- Air brake systems have an inherent brake lag component.
Memory trick: Perceiving, Reacting, Lagging, Braking – that's the full distance.