A commercial motor vehicle driver is descending a steep, five-mile mountain downgrade with… — Commercial Driver's License (CDL) General Knowledge Exam

A commercial motor vehicle driver is descending a steep, five-mile mountain downgrade with a fully loaded combination vehicle. After shifting into an appropriate low gear, the driver establishes a safe maximum speed of $35\text{ mph}$ . When the vehicle reaches $35\text{ mph}$ , which procedure should the driver use to control speed and prevent brake fade?

A
Maintain continuous, light pedal pressure on the service brakes to hold the vehicle at a steady $35\text{ mph}$ throughout the entire descent.
Apply the brakes firmly to slow the vehicle to $30\text{ mph}$ in about 3 seconds, release the brakes completely, and repeat once speed climbs back to $35\text{ mph}$ .Answer
C
Coast in neutral and apply maximum braking force only after entering curves along the downgrade to scrub off excess momentum.
D
Engage auxiliary retarders at maximum power on slick surfaces and pump the service brake pedal rapidly to counteract brake lag.

Answer

Apply the brakes firmly to reduce speed to

30\text{ mph}

in approximately 3 seconds, release the brakes completely, and repeat the application whenever vehicle speed returns to

35\text{ mph}

The proper technique for controlling speed on steep downgrades is snub braking. Once the vehicle reaches the maximum safe speed in a low gear, the driver applies the brakes firmly enough to slow the vehicle to

5\text{ mph}

below the safe speed in about 3 seconds, then releases the pedal completely. This allows the brake drums and linings adequate cooling intervals between applications, preventing brake fade.

Step-by-Step Solution

Select the correct gear and establish the designated safe speed for the downgrade before starting the descent.

Engine compression provides primary retarding force, keeping vehicle speed near the target

35\text{ mph}

Engine braking reduces the amount of mechanical friction braking required from the foundation brakes.

Apply service brakes with enough force to feel a definite slowdown until the vehicle speed drops

5\text{ mph}

below the safe speed (to

30\text{ mph}

Vehicle speed decreases from

35\text{ mph}

30\text{ mph}

in approximately 3 seconds.

Firm, short-duration brake application creates sufficient retarding force without excessively overheating the brake drums.

Release the service brakes completely once speed drops to

30\text{ mph}

and allow the vehicle speed to gradually build back up to

35\text{ mph}

Air flow cools the brake components while the vehicle speed slowly increases.

Releasing the brakes provides a cooling window, preventing thermal expansion, glaze formation, and catastrophic brake fade.

Key Concept

Snub braking technique for downhill speed control and brake fade prevention

Estimated Time:1m 15s

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