CDL General Knowledge Test · CDL General Knowledge Test
CDL General Knowledge Test: Speed and Space Management Concepts
Importance ★★★★★ · 14% (estimate)
Written by AI. Only units that passed a separate AI check (re-reading facts and numbers) are published. Tell us if you find a mistake.
Overview
This chapter covers how speed and space keep a large vehicle out of trouble. Driving too fast is a major cause of fatal crashes, and heavy vehicles need much more room to stop and turn than cars do.
Total stopping distance is perception distance plus reaction distance plus braking distance. When you double your speed, braking distance becomes about four times longer. Wet roads can double stopping distance, so cut your speed by about one-third on a wet road and by half or more on packed snow. If you hydroplane, release the accelerator and push in the clutch, and do not brake. Empty trucks can take longer to stop because they have less traction.
Keep at least one second of following distance for every 10 feet of vehicle length below 40 mph, and add one more second above 40 mph. If someone tailgates you, add space in front and slow down gradually. Watch the space overhead, since an empty van sits higher than a loaded one. On right turns, keep the rear near the curb and turn wide at the end, and with two left-turn lanes, use the outside one.
Key concepts
Total stopping distance
Perception distance (until your brain recognizes the hazard) plus reaction distance (until your foot hits the brake) plus braking distance (until the vehicle stops).
Memory tipSee it, react, brake: three distances add up.
Speed and braking distance
Braking distance grows much faster than speed. Doubling your speed from 20 to 40 mph makes the braking distance and the force of impact about four times greater.
Memory tipTwice the speed, four times the distance.
Source: New York State DMV Commercial Driver's Manual (CDL-10), Section 2 · as of 2026
Following distance formula
Keep at least 1 second for each 10 feet of vehicle length below 40 mph. Above 40 mph, add 1 more second. A 40-foot truck needs 4 seconds below 40 mph and 5 seconds above it.
Memory tipLength divided by 10, plus 1 at highway speed.
Source: New York State DMV Commercial Driver's Manual (CDL-10), Section 2 · as of 2026
Slippery road speeds
On a wet road, slow down by about one-third, for example from 55 to about 35 mph. On packed snow, slow by half or more. On ice, slow to a crawl and stop as soon as you safely can.
Memory tipWet: one-third off. Snow: half off. Ice: crawl.
Source: New York State DMV Commercial Driver's Manual (CDL-10), Section 2 · as of 2026
Hydroplaning
Tires ride on a layer of water and lose contact with the road. It can happen as low as 30 mph with a lot of water, and is more likely with low tire pressure or worn tread. Release the accelerator and push in the clutch. Do not brake.
Memory tipLike water skiing: ease off and let the wheels roll.
Source: New York State DMV Commercial Driver's Manual (CDL-10), Section 2 · as of 2026
Turning space
On a right turn, keep the rear close to the curb and turn wide as you finish, so no one passes on your right. With two left-turn lanes, use the outside (right-hand) one.
Memory tipDo not swing left before a right turn.
Often tested
- Total stopping distance = perception + reaction + braking distance.
- Double the speed and braking distance becomes about four times longer.
- Following distance: 1 second per 10 feet of length, plus 1 second above 40 mph.
- If you are tailgated, add space in front and slow down gradually.
- An empty van is taller than a loaded one, so check bridge clearance again.
- Empty trucks can need more distance to stop than loaded trucks.
3 sample questions
Verified practice questions from this unit.
Question 1. What three distances add up to total stopping distance?
- ① Perception distance, reaction distance, and braking distance
- ② Reaction distance, following distance, and braking distance
- ③ Braking distance only
▼ Show answer and explanation▲ Hide answer and explanation
Answer: ① Perception distance, reaction distance, and braking distance
Key point: Perception + reaction + braking = stopping
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.
Question 2. If you double your speed from 20 to 40 mph, what happens to your braking distance?
- ① It becomes about twice as long
- ② It becomes about four times longer
- ③ It stays about the same
▼ Show answer and explanation▲ Hide answer and explanation
Answer: ② It becomes about four times longer
Key point: Double the speed = 4 times the braking distance
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.
Question 3. How much should you slow down on a wet road?
- ① By about one-half or more
- ② By about 5 mph
- ③ By about one-third (for example, from 55 mph to about 35 mph)
▼ Show answer and explanation▲ Hide answer and explanation
Answer: ③ By about one-third (for example, from 55 mph to about 35 mph)
Key point: Wet road: cut speed by about one-third
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.