Normal Driving with Air Brakes and Stopping Distance
Scope
Normal Driving with Air Brakes and Stopping Distance tests how air-brake lag, speed, weight, and road surface change stopping distance, and how to brake without overheating the system. Expect questions on controlled braking, stab braking, following distance, and downgrades.
Core rules
- Air brakes do not apply instantly. At 55 mph, brake lag alone accounts for about 32 feet; total stopping distance for a loaded tractor-trailer is about 293 feet.
- Total stopping distance includes perception, reaction, brake lag, and actual braking. Speed affects braking distance much more than it affects reaction distance.
- Use controlled braking: press hard enough to slow or stop without locking the wheels, so you can still steer.
- Use stab braking only if wheels lock: apply fully, release when the wheels roll, then reapply.
- Before a downgrade, select a lower gear. Use engine braking and short, controlled brake applications; do not ride the brakes.
- Increase following distance with speed, weight, and wet or icy pavement.
Worked example
At 55 mph, a hazard appears 250 feet ahead. The vehicle needs about 293 feet to stop, so 250 feet is not enough even with good brakes. The driver should have reduced speed and increased following distance before the hazard. At 60 mph the gap would be worse, because braking distance rises with the square of speed.
Traps
Do not treat air brakes as instantaneous. Do not pump the pedal in normal driving. Do not use stab braking as a routine technique; it is for wheel lock. Do not coast downhill in neutral to save brakes.