CDL General Knowledge Test Speed and Space Management Summary

Total stopping distance = perception distance + reaction distance + braking distance. You keep moving while your brain recognizes the hazard and while your foot moves to the brake.

Wrong choices

• Reaction distance, following distance, and braking distance: Following distance is the gap you keep, not part of stopping.
• Braking distance only: You travel a long way before the brakes even start working.

When you double your speed, the impact and the braking distance become about four times greater. Triple the speed and they become about nine times greater.

Wrong choices

• It becomes about twice as long: Braking distance grows much faster than speed.
• It stays about the same: Speed has a big effect on braking distance.

Wet roads can double stopping distance. Reduce speed by about one-third on a wet road (for example, from 55 to about 35 mph). On packed snow, slow by one-half or more. On ice, slow to a crawl and stop as soon as you safely can.

Wrong choices

• By about one-half or more: That is the rule for packed snow.
• By about 5 mph: 5 mph is not enough. Wet roads can double stopping distance.

When you hydroplane, the tires ride on water and lose traction. Release the accelerator and push in the clutch to slow down and let the wheels turn freely. Do not use the brakes.

Wrong choices

• Brake firmly to slow down: Do not use the brakes while hydroplaning.
• Speed up to get through the water faster: More speed makes hydroplaning worse.

Use one second for every 10 feet of vehicle length below 40 mph.

1. Vehicle length: 40 feet ÷ 10 = 4 seconds
2. Speed is 35 mph, which is below 40 mph, so do not add a second.
3. Following distance = 4 seconds

Wrong choices

• 5 seconds: You add the extra second only above 40 mph.
• 2 seconds: 2 seconds is too close for a 40-foot vehicle.

Use one second for every 10 feet of length, then add one second above 40 mph.

1. Vehicle length: 60 feet ÷ 10 = 6 seconds
2. Speed is 55 mph, above 40 mph, so add 1 second.
3. 6 + 1 = 7 seconds

Wrong choices

• 6 seconds: 6 seconds is right only below 40 mph. Add 1 second above 40 mph.
• 8 seconds: You add one extra second for highway speed, not two.

Turn slowly. Keep the rear of the vehicle close to the curb and turn wide as you complete the turn. This stops drivers from passing you on the right. Do not swing wide to the left first, because a driver may slip in on your right.

Wrong choices

• Swing wide to the left before starting the turn: A driver behind may think you are turning left and try to pass you on the right.
• Back up to make room if a car comes toward you: Do not back up. You could hit someone behind you.

A heavier vehicle makes the brakes work harder. But truck brakes, tires, springs, and shocks are designed to work best when fully loaded. Empty trucks can need greater stopping distances because they have less traction.

Wrong choices

• A fully loaded truck, because the tires grip less: Brakes, tires, and springs work best when the truck is loaded.
• Neither. Weight has no effect on stopping: Weight does affect stopping.

The weight of a cargo van changes its height. An empty van is higher than a loaded one. Getting under a bridge loaded does not mean you can do it empty. Also, posted heights may be wrong after repaving or packed snow.

Wrong choices

• An empty van sits lower, so it will clear more easily: It is the other way around. Weight pushes the van down.
• Cargo weight does not change the van's height: Weight does change the height.

If you are tailgated, open up more space in front of you so you will not have to stop suddenly, and signal early and slow down very gradually. Do not speed up, and do not flash your lights or brake lights to send a message.

Wrong choices

• Flash your brake lights to make them back off: The manual says to avoid tricks like this.
• Speed up to get away from them: It is safer to be tailgated at a low speed than a high speed.