Speed & Space Management

Study questions with answers

12 sample Speed & Space Management questions with the correct answer, a short explanation, and the official handbook reference. Read through them, then take the quiz above.

1. 1. What should be the main way you control your speed when going down a long, steep grade?

   Correct answer: The braking effect of the engine

   The braking effect of the engine should be your primary means of controlling speed on a downgrade. The service brakes only supplement that engine braking.

   Source: Illinois CDL Guide — Sec. 2.6, Speed on Downgrades

2. 2. Why does your vehicle's speed increase on a downgrade?

   Correct answer: Gravity pulls the vehicle downhill

   Gravity pulls the vehicle downhill, so its speed naturally increases on a downgrade. That is why you must control speed with engine braking and be in the proper low gear.

   Source: Illinois CDL Guide — Sec. 2.6, Speed on Downgrades

3. 3. About how far will a heavy vehicle travel to stop from 55 mph on dry pavement under ideal conditions?

   Correct answer: About 419 feet

   Adding perception, reaction, and braking distances, a vehicle at 55 mph needs roughly 419 feet to stop under ideal conditions — a distance longer than a football field.

   Source: Illinois CDL Guide — Sec. 2.6, Stopping Distance

4. 4. Why must you be especially aware of your vehicle's size when crossing or entering traffic?

   Correct answer: It accelerates slowly and needs a larger gap

   A large, heavy vehicle needs a much bigger gap than a car to cross or merge, because it accelerates slowly and takes longer to clear the roadway.

   Source: Illinois CDL Guide — Sec. 2.7, Space to Cross or Enter Traffic

5. 5. Why can a truck with a high center of gravity be especially dangerous in a curve?

   Correct answer: It can roll over even at the posted curve speed

   Testing shows that trucks carrying their weight up high can tip over even while traveling at the posted speed for a curve, so drivers should slow well below that limit.

   Source: Illinois CDL Guide — Sec. 2.6, Speed and Curves

6. 6. Using the length rule, how much following distance does a 40-foot vehicle need at speeds below 40 mph?

   Correct answer: Four seconds

   The rule allows roughly one second of space for each 10 feet of vehicle length below 40 mph. A 40-foot vehicle therefore needs about four seconds of following distance.

   Source: Illinois CDL Guide — Sec. 2.7, Space Ahead

7. 7. Total stopping distance for a commercial vehicle is made up of which three parts?

   Correct answer: Perception, reaction, and braking distance

   Total stopping distance adds together perception distance (while you notice the hazard), reaction distance (while you move your foot to the brake), and braking distance (while the brakes bring you to a stop).

   Source: Illinois CDL Guide — Sec. 2.6, Stopping Distance

8. 8. At what speed can hydroplaning begin if there is a lot of water on the road?

   Correct answer: As low as about 30 mph

   It does not take much water for tires to lose contact with the road. With enough standing water, hydroplaning can start at speeds as low as about 30 mph.

   Source: Illinois CDL Guide — Sec. 2.6, Hydroplaning

9. 9. Why does an empty truck often need a longer stopping distance than a loaded one?

   Correct answer: An empty vehicle has less traction

   The brakes, tires, and suspension of a heavy vehicle are designed to work best when it is loaded. Empty, the vehicle has less traction, so it can actually take longer to stop.

   Source: Illinois CDL Guide — Sec. 2.6, Effect of Weight

10. 10. How does the following-distance rule change once you are traveling above 40 mph?

    Correct answer: You add one second for safety

    At speeds over 40 mph you add one extra second to the length-based figure. A 40-foot vehicle that needs four seconds below 40 mph should be given five seconds above it.

    Source: Illinois CDL Guide — Sec. 2.7, Space Ahead

11. 11. At speeds below 40 mph, how much following distance should a 60-foot combination allow?

    Correct answer: Six seconds

    The rule allows roughly one second of space per 10 feet of vehicle length below 40 mph, so a 60-foot rig needs about six seconds, and seven seconds once above 40 mph.

    Source: Illinois CDL Guide — Sec. 2.7, Space Ahead

12. 12. Why is overhead clearance a concern you must plan for with a tall vehicle?

    Correct answer: Repaving or packed snow can reduce a posted clearance

    Striking overhead objects is a real danger. Repaving or packed snow can lower posted clearances, so make sure you always have room overhead in the path you will take.

    Source: Illinois CDL Guide — Sec. 2.7, Space Overhead