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Question

Difficulty: MediumMountain Driving and Steep Downgrades

While navigating a steep 6%6\% mountain descent on wet pavement with a loaded combination vehicle, a commercial driver notes that road momentum has brought vehicle speed up to the established safe limit of 45 mph45\text{ mph}. What is the correct procedure for the driver to manage speed while preventing drive-wheel skids and thermal brake fade?

  1. A
    Engage the auxiliary engine retarder at its maximum setting to maintain speed control without applying the foundation service brakes on the wet surface.
  2. B
    Maintain continuous, light pedal pressure on the foundation brakes to hold vehicle speed steadily at 45 mph45\text{ mph} throughout the descent.
  3. Ensure auxiliary retarders are switched off, then apply the service brakes firmly to reduce speed to 40 mph40\text{ mph} before releasing the pedal completely.Answer
  4. D
    Place the transmission into neutral and pump the brake pedal with rapid, light taps whenever the speedometer exceeds 45 mph45\text{ mph}.

Answer

The driver must turn off auxiliary retarders to prevent skidding on wet pavement and perform proper snub braking by braking firmly until vehicle speed drops to 5 mph5\text{ mph} below the safe speed (40 mph40\text{ mph}) and then fully releasing the brake pedal.
On a steep downgrade in wet conditions, auxiliary retarders must be disabled because their braking torque can break drive-tire traction and induce a skid. Foundation speed control must rely on the snub braking technique: once vehicle speed reaches the safe speed (45 mph45\text{ mph}), the driver applies firm service braking until speed drops approximately 5 mph5\text{ mph} below that threshold (40 mph40\text{ mph}) and then completely releases the pedal, allowing the brakes to cool between cycles.

Step-by-Step Solution

1
Assess road surface conditions and auxiliary retarder status.
Confirm that the road is wet and that all engine/exhaust/driveline retarders are deactivated.
Retarder braking force applied exclusively to drive wheels on slippery surfaces can immediately induce wheel lockup and jackknifing.
2
Initiate foundation service brake application when reaching safe speed.
Apply firm, steady pedal pressure to feel a definite slowdown from 45 mph45\text{ mph} down to 40 mph40\text{ mph} (lasting roughly three seconds).
Firm application provides positive deceleration while the vehicle remains in the preselected proper low gear.
3
Release the foundation brakes completely once speed drops to 40 mph40\text{ mph}.
Allow vehicle speed to naturally increase back toward 45 mph45\text{ mph} under engine braking resistance before repeating the cycle.
Complete release allows brake drums and linings to dissipate friction heat into the surrounding airflow, preventing mechanical brake fade.

Key Concept

Steep Downgrade Speed Management and Wet Weather Retarder Safety
Estimated Time:1m 0s
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Topics

Mountain Driving and Steep Downgrades

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