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Difficulty: Very hardMountain Driving and Steep Downgrades

You are driving a commercial motor vehicle with a gross combination weight of 78,000 lbs. You are approaching the crest of a 6-mile, 6% downgrade. The road surface is covered in freezing rain and packed snow. The posted safe speed for this downgrade for your vehicle's weight is 25 mph. Your vehicle is equipped with a multi-stage engine retarder. Which of the following describes the correct procedure for managing your speed on this downgrade?

  1. Turn the engine retarder completely OFF. Shift into an appropriate low gear before starting down the grade. Once your vehicle reaches 25 mph, apply the service brakes firmly to reduce your speed to 20 mph, then release the brakes entirely.Answer
  2. B
    Set the engine retarder to its lowest stage to provide supplemental braking while minimizing skid risk. Shift into an appropriate low gear before starting down the grade. Once your vehicle reaches 25 mph, apply the service brakes firmly to reduce your speed to 20 mph, then release the brakes.
  3. C
    Turn the engine retarder completely OFF. Shift into an appropriate low gear before starting down the grade. Once your vehicle reaches 25 mph, apply steady, light, continuous pressure to the service brakes to maintain exactly 25 mph throughout the descent.
  4. D
    Turn the engine retarder completely OFF. Shift into an appropriate low gear before starting down the grade. Allow gravity to accelerate your vehicle to 30 mph, then apply the service brakes firmly to reduce your speed back to the safe speed of 25 mph, and release.

Answer

Turn the engine retarder completely OFF, shift into an appropriate low gear before starting down the grade, and use the proper snub braking technique by firmly applying the brakes at 25 mph to reduce your speed to 20 mph before releasing them.
The correct answer accurately applies three distinct rules for commercial vehicles on mountain downgrades. First, because the road is covered in freezing rain and snow, the engine retarder must be turned off entirely to prevent a loss of traction on the drive wheels. Second, safety regulations require shifting into a lower gear before beginning the descent, relying primarily on engine braking. Third, it correctly identifies the mechanical snub braking formula: wait until gravity pushes the vehicle to its maximum safe speed (25 mph), apply the brakes firmly enough to feel a definite slowdown, and release them once the speed drops by 5 mph (20 mph).

Step-by-Step Solution

1
Analyze the road conditions and apply retarder rules.
The road is covered in freezing rain and packed snow, so the engine retarder must be turned completely OFF.
Using an engine retarder on slick surfaces can cause the drive wheels to lock up and initiate a dangerous skid.
2
Determine the proper timing for gear selection.
You must shift into a low gear before beginning the descent.
Shifting gears while already descending a steep grade is strictly prohibited because you may not be able to get the transmission back into gear once it is in neutral.
3
Calculate the correct speeds for the snub braking technique.
Braking should begin at 25 mph and continue until the vehicle reaches 20 mph.
Proper snub braking requires applying the brakes firmly when the vehicle reaches the posted safe speed, reducing the speed to 5 mph below that safe speed, and then releasing the brakes to allow them to cool.

Key Concept

Synthesizing Mountain Driving Safety Protocols (Retarders, Gear Selection, and Snub Braking)
Estimated Time:2m 0s
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Topics

Mountain Driving and Steep Downgrades

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