Speed Management: Practice Questions & Explanations

On a wet road, you should reduce your speed by about one-third compared to dry conditions. So if traffic and conditions on a dry road would have you at 55 mph, on wet pavement you would be down around 35 mph. On packed snow, reduce by one-half. On ice, reduce by two-thirds. The reasons are stopping distance and traction: wet pavement can double stopping distance, and a heavy vehicle hydroplanes more easily than a car. Hydroplaning can begin at speeds as low as 30 mph when there is enough water on the road, and is more likely with worn tires, underinflated tires, or in puddles. Driving below the posted limit when conditions require it is not only legal, it is mandatory under the basic speed rule that applies in every state.

Stopping distance increases with the square of speed, not linearly. Doubling speed roughly quadruples the distance needed to stop. This is because braking energy is proportional to the square of speed, so twice as fast means four times as much kinetic energy that the brakes must dissipate. Total stopping distance has three components: perception distance (how far you travel while recognizing the hazard), reaction distance (how far you travel while moving your foot to the brake), and braking distance. Perception and reaction distances scale linearly with speed; braking distance scales with the square. The implication: small speed reductions produce disproportionately large safety gains, and high-speed crashes are far more violent than the speed difference alone suggests.

BRAKE FADE is the reduction in braking effectiveness caused by overheating brake components. HOW IT HAPPENS: When a brake pad or shoe presses against a drum or rotor, friction converts kinetic energy to heat. If brakes are applied continuously for too long (as on a long mountain descent), the drum/rotor and friction material absorb more heat than can be dissipated to the air — the surface temperature rises to 400-600°F+. At these temperatures, the friction material (brake lining) undergoes chemical changes that reduce its coefficient of friction — it literally loses the ability to grip. FADE INDICATORS: Increasing pedal travel needed for same stopping force; brakes feeling 'soft' or 'spongy'; burning smell; reduced effectiveness in emergency stops. TWO TYPES: (1) THERMAL FADE — hot friction material loses gripping ability; occurs from sustained application as described; (2) VAPOR LOCK (for hydraulic brakes) — brake fluid boils, gas bubbles compress instead of transmitting pressure. PREVENTION: Use engine braking (downshift on grades) instead of riding friction brakes; use the stab braking technique if brakes are hot (press hard, release, press again); know where escape ramps are. RECOVERY: Stop and let brakes cool (15-30 minutes); do NOT continue driving with faded brakes — they won't recover while still hot and being used. This is why FMCSA regulations require CMV drivers to test brakes before long descents.