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Difficulty: HardSpeed and Road Conditions

A commercial driver is navigating a tractor-trailer along a rural two-lane route posted at 50 mph50\text{ mph}. The highway surface is covered with packed snow, and a downhill curve is approaching. Which operational procedure should the driver follow to safely negotiate the road conditions?

  1. A
    Keep the engine retarder activated on its highest setting to assist service braking, slow to 35 mph35\text{ mph} before the curve, and apply continuous light braking throughout the turn.
  2. Turn off the engine retarder, reduce speed to 25 mph25\text{ mph} or less before entering the curve, and avoid applying brakes while turning.Answer
  3. C
    Maintain 35 mph35\text{ mph} on the straight approach, disengage the retarder, and apply firm service braking once inside the curve to bring the vehicle down to 25 mph25\text{ mph}.
  4. D
    Reduce speed by one-third to approximately 33 mph33\text{ mph} using service brakes on approach, leaving the following distance unchanged under the assumption that total stopping distance is unaffected.

Answer

The driver should turn off the engine retarder, reduce vehicle speed to 25 mph25\text{ mph} or less prior to entering the curve, and refrain from braking while navigating the turn.
On packed snow, commercial drivers must reduce speed by at least one-half (from 50 mph50\text{ mph} down to 25 mph25\text{ mph} or less). Furthermore, engine retarders must be switched off on slippery roads to avoid drive-wheel lockup and jackknifing, and all deceleration must be completed on the straightaway before entering the curve so tire traction is reserved exclusively for steering.

Step-by-Step Solution

1
Calculate the mandatory speed reduction for packed snow conditions.
On packed snow, CDL guidelines require reducing speed by at least one-half (1/21/2). From a base posted speed of 50 mph50\text{ mph}, safe travel speed is 25 mph25\text{ mph} or lower.
Packed snow drastically lowers the tire-to-road friction coefficient, multiplying stopping distance.
2
Adjust auxiliary braking systems for slippery roadway conditions.
Turn off the engine retarder / engine brake.
Retarders deliver braking force solely to the drive axles; on slippery surfaces like packed snow, this can cause drive-wheel lockup and an immediate jackknife.
3
Plan the deceleration zone before negotiating the curve on a downgrade.
Complete all necessary slowing before the vehicle enters the curve, avoiding brake application inside the turn.
Tires have limited traction; splitting available traction between steering forces and braking forces inside a slippery turn easily exceeds friction limits and leads to a spinout or skid.

Key Concept

Speed reduction ratios, auxiliary retarder safety, and curve management on adverse roadway surfaces
Estimated Time:1m 30s
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Topics

Speed and Road Conditions

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