Missouri CDL — Air Brakes practice
Speed & Space Management
Choosing a safe speed, keeping a space cushion, adjusting for weather and traffic, and understanding stopping distance.
Questions reviewed against the official Missouri driver handbook · July 7, 2026
5 questions · pass with 4 correct. You get instant feedback and an explanation after every answer.
Study questions with answers
5 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. On a long, steep downgrade, the service brakes should be used how?
Correct answer: As a supplement to the engine's braking, after you are in a low gear
On a downgrade the brakes only back up the engine's braking effect; select the proper low gear first, then brake in short applications.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.6 Proper Braking Technique
2. Why do air brakes have 'brake lag' that hydraulic brakes do not?
Correct answer: It takes a moment for the air to travel through the lines to the brakes
Unlike hydraulic brakes that act instantly, air brakes need a fraction of a second for air to move through the lines, adding brake-lag distance.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.4 Stopping Distance
3. The total stopping distance for an air brake vehicle is made up of which four parts?
Correct answer: Perception, reaction, brake lag, and braking distance
For air brakes, the total stopping distance combines perception distance, reaction distance, brake-lag distance, and braking distance.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.4 Stopping Distance
4. At 55 mph on dry pavement, about how much distance does air brake lag add?
Correct answer: About 32 feet
Brake lag at 55 mph on dry pavement adds roughly 32 feet, contributing to a total stopping distance of over 450 feet.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.4 Stopping Distance
5. In the recommended downgrade braking pattern, once your speed is about 5 mph below your safe speed, what do you do?
Correct answer: Release the brakes until your speed climbs back to the safe speed, then repeat
Apply the brakes hard enough to feel a definite slowdown, then let off once you are about 5 mph under your safe speed, repeating the cycle down the grade.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.6 Proper Braking Technique
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Every Missouri question is written from the official Missouri driver handbook and checked against its current edition. DMV Test Free is a free, independent study resource — not affiliated with any DMV or government agency. About DMV Test Free