Speed Management and Stopping Distance
Speed Management and Stopping Distance
This domain tests whether you understand how speed, weight, road condition, and reaction time combine to lengthen stopping distance. It also tests choosing a safe speed before curves, ramps, and downgrades.
Core rules
- Total stopping distance has perception distance, reaction distance, brake lag, and braking distance. perception and reaction occur before the brakes apply; brake lag is the time for air pressure to reach the brakes; braking distance is the actual slowing distance.
- Braking distance increases with the square of speed. Doubling speed roughly quadruples braking distance, while reaction distance only doubles. Wet, icy, or gravel surfaces reduce traction and increase distance further.
- Slow down before a curve, not in it. On a downgrade, select a low enough gear before the descent and use engine braking; do not ride the brakes until they fade.
Worked example
If braking distance at 20 mph is about 40 feet, at 40 mph it is about 160 feet because 40 divided by 20 is 2, and 2 squared is 4. Reaction distance only doubles. Reasoning: the total stopping distance grows much faster than speed, so entering a curve or work zone at double speed leaves far less margin than drivers expect.
Traps that cost marks
Assuming stopping distance rises in a straight line with speed; braking late into a curve; relying on brakes instead of gearing on a long downgrade; and ignoring how load or wet pavement changes stopping distance.