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Difficulty: Very hardSpeed on Curves and Downgrades

You are operating a fully loaded combination vehicle and are preparing to descend a 6-mile, 7% mountain grade. The road surface is partially covered in packed snow. A warning sign at the crest indicates a sharp curve two miles down the grade with an advisory speed of 25 mph. Which of the following describes the strictly correct and safest procedure for managing your speed and vehicle control throughout this descent?

  1. Turn off the engine retarder. Before beginning the descent, downshift into a gear low enough to hold your speed at 25 mph. Whenever gravity pushes your speed to 25 mph, firmly apply the service brakes to reduce your speed to 20 mph, then release them, ensuring you are at or below 25 mph before entering the curve.Answer
  2. B
    Shift into a low gear before the descent and engage the engine retarder to its highest setting to minimize service brake wear. Allow the retarder to hold the vehicle at 25 mph, applying light service brake pressure only while steering through the apex of the curve.
  3. C
    Select a low gear before starting the descent. Maintain a light, continuous drag on the service brakes to hold the vehicle at exactly 25 mph for the entire 6-mile downgrade, keeping this steady brake pressure applied to maximize stability as you navigate through the curve.
  4. D
    Begin the descent in a higher gear at 40 mph to maintain momentum on the straight sections. Upon approaching the curve, apply firm, continuous braking to bring your speed down to the 25 mph limit just as you enter the curve, then downshift to a lower gear.

Answer

The correct procedure requires turning off the engine retarder, selecting a low gear before the descent, and using the proper snub braking technique (braking from 25 mph to 20 mph) to ensure you enter the curve at a safe speed.
The correct answer safely synthesizes all required protocols for this high-risk scenario. It acknowledges that the engine retarder must be disabled due to the snow to prevent a skid. It correctly identifies that the driver must select a gear suited for the 25 mph curve before starting the descent. Finally, it applies the exact snub braking technique defined in the CDL manual (let speed build to safe speed, brake hard to 5 mph below, release) and ensures all slowing down happens before the curve.

Step-by-Step Solution

1
Analyze the road conditions and equipment.
The road is covered in packed snow, meaning traction is poor.
Engine retarders can cause the drive wheels to lock up and induce a skid on slippery surfaces, so the retarder must be turned off.
2
Determine the target safe speed for the descent.
The lowest required speed is 25 mph due to the sharp curve ahead.
You must select a gear low enough to maintain this speed before crossing the crest of the hill. Downshifting midway down a steep grade is extremely dangerous.
3
Execute proper speed management using the service brakes.
Use the snub braking method: let the vehicle reach the target safe speed (25 mph), brake firmly to 5 mph below that (20 mph), and release.
This technique prevents brake fade by allowing the brakes to cool between applications, unlike continuous light dragging.
4
Navigate the curve.
Complete all necessary braking before entering the curve.
Braking mid-curve shifts the vehicle's center of gravity and reduces the tires' lateral grip, increasing the risk of a skid or rollover.

Key Concept

Synthesizing proper gear selection, engine retarder limitations in poor weather, the snub braking technique, and curve entry physics.
Estimated Time:2m 0s
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Topics

Speed on Curves and Downgrades

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