A fully loaded commercial vehicle with air brakes is traveling at 55 mph. The driver needs to make an emergency stop on dry pavement. Assuming good brakes and driver reaction, approximately how far will the vehicle travel from the moment the driver perceives the need to stop until the vehicle comes to a complete halt?
- AApproximately 200 feet.
- BApproximately 300 feet.
- CApproximately 450 feet.
- DApproximately 150 feet.
Show answer & explanationAnswer & explanation
Correct answer: C. Approximately 450 feet.
At 55 mph, a fully loaded commercial vehicle with air brakes will travel approximately 450 feet to stop. This includes perception distance (about 4 seconds or 330 ft at 55 mph), reaction distance (3/4 second or 60 ft), and braking distance (which for air brakes can be around 250 ft, but the 450 ft figure is a common safety estimate for total stopping distance at 55 mph, considering brake lag and typical conditions). The rule of thumb for air brakes is often cited as 450 feet at 55 mph.
Why the other options are wrong
- A. 200 feet is also significantly underestimated for a commercial vehicle at this speed.
- B. 300 feet is still too short; while closer, it doesn't account for the full perception, reaction, and air brake lag components.
- D. 150 feet is far too short for a fully loaded commercial vehicle with air brakes at 55 mph; this might be closer to a car's stopping distance.
Total Stopping Distance (Commercial Vehicle at 55 mph)
The total distance a fully loaded commercial vehicle with air brakes travels from the moment a driver perceives a hazard until the vehicle comes to a complete stop.
- Includes perception, reaction, brake lag, and actual braking distance.
- Approximately 450 feet at 55 mph.
- Significantly longer than passenger vehicles.
- Crucial for safe following distance.
Memory trick: Fifty-five, four hundred fifty, stay alive and don't be hasty.