Speed Management and Stopping Distance
This domain tests whether you understand what actually determines stopping distance and how to choose a speed for conditions, not for the posted sign. Expect questions that ask you to add up perception, reaction, brake lag, and braking distance, or to pick a safe speed on a descent or in rain.
The four parts of stopping distance
- Perception distance: the time to notice a hazard, about 3/4 second.
- Reaction distance: the time to move your foot, about 3/4 second.
- Brake lag: the time for air to reach the chambers, roughly half a second, or about 30-40 feet at highway speed with air brakes.
- Braking distance: the actual slowing distance, which grows with the square of speed.
Speed choice
- Never drive faster than conditions allow; the basic speed law overrides the posted limit. In the District, the default limit is 25 mph unless posted otherwise, and many truck routes carry lower posted limits.
- Reduce speed by about one-third on wet pavement, by half or more on packed snow, and to a crawl on ice.
- Before a downgrade, select a gear that holds your speed and use the brakes lightly and steadily rather than riding them.
- Enter curves below the advisory speed; a loaded trailer resists direction change and leans outward.
Worked example
At 55 mph a loaded combination needs roughly 420 feet to stop: about 60 feet of perception, 60 feet of reaction, roughly 32 feet of air-brake lag, and over 200 feet of braking. Reasoning: if the driver doubles speed, reaction distance doubles but braking distance roughly quadruples because it is proportional to the square of velocity. That is why 30 mph feels safe and 60 mph does not, even with identical brakes.
Traps
- Treating total stopping distance as braking distance alone.
- Believing ABS shortens stopping distance on every surface; on loose gravel it may lengthen it.
- Using the posted limit as proof of a safe speed in rain or traffic.
- Ignoring brake lag on a vehicle with air brakes.