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Question

Difficulty: MediumSpeed and Road Conditions

A commercial motor vehicle is traveling on a rural highway where the posted dry-road speed limit is 60 mph60\text{ mph}. As a winter storm develops, the roadway becomes covered with packed snow. Which of the following procedures should the driver follow to maintain safe vehicle control?

  1. A
    Reduce vehicle speed to 30 mph30\text{ mph} or less while engaging the engine retarder to maximum power to assist the service brakes.
  2. B
    Maintain the posted speed on straight sections and brake firmly after entering sharp curves to maintain lateral stability.
  3. Reduce vehicle speed to 30 mph30\text{ mph} or less and turn off the engine retarder.Answer
  4. D
    Reduce vehicle speed to 40 mph40\text{ mph} under the assumption that only the mechanical braking distance increases on snow.

Answer

Reduce vehicle speed to 30 mph30\text{ mph} or less and turn off the engine retarder.
According to commercial driving safety guidelines, a driver operating on packed snow must reduce speed by at least one-half (from 60 mph60\text{ mph} to 30 mph30\text{ mph} or less). Additionally, engine retarders must always be disabled on wet, icy, or snow-covered surfaces because auxiliary braking on the drive axle can induce an uncontrollable skid.

Step-by-Step Solution

1
Determine the required speed reduction proportion for packed snow.
Official CDL standards specify reducing speed by at least one-half (1/21/2) on packed snow. From a base speed of 60 mph60\text{ mph}, the vehicle should slow to 30 mph30\text{ mph} or below.
Packed snow drastically reduces tire-road traction, requiring a major reduction in operating speed to maintain steering and braking capability.
2
Configure the engine retarder appropriately for slippery surface conditions.
Switch off or disable the engine retarder.
Engine retarders apply braking torque directly to the drive axles, which can cause the drive wheels to lose traction and initiate a jackknife or skid on slippery surfaces.

Key Concept

Speed reduction rules and auxiliary braking management on slippery road surfaces
Estimated Time:1m 0s
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Topics

Speed and Road Conditions

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