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Difficulty: MediumSpeed and Road Conditions

A commercial motor vehicle is traveling on a secondary highway with a posted speed limit of 35 mph35\text{ mph}. As the vehicle approaches a shaded bridge deck that has frozen over with glare ice, which combination of speed adjustment and vehicle control actions should the driver perform?

  1. A
    Reduce speed by one-third to approximately 23 mph23\text{ mph} while keeping the engine retarder engaged at full strength to assist deceleration.
  2. B
    Reduce speed by only 5 mph5\text{ mph} based on the assumption that stopping distance on ice is limited solely to the vehicle's mechanical braking distance.
  3. Slow the vehicle down to a crawl (approximately 5 to 15 mph5\text{ to }15\text{ mph}) and turn off the engine retarder before crossing the icy surface.Answer
  4. D
    Maintain the posted 35 mph35\text{ mph} speed across the bridge deck and apply the service brakes firmly after entering the downstream curve.

Answer

Slow the vehicle down to a crawl (approximately 5 to 15 mph5\text{ to }15\text{ mph}) and turn off the engine retarder before crossing the icy surface.
Commercial driving safety standards state that when encountering packed ice or glare ice, drivers must reduce their speed to a crawl (5 to 15 mph5\text{ to }15\text{ mph}) to maintain control. In addition, engine retarders must be disabled whenever road surfaces are wet, icy, or snow-covered because auxiliary braking forces applied to drive wheels can break traction and precipitate an uncontrollable skid.

Step-by-Step Solution

1
Determine the required speed reduction for the specific road surface condition.
On solid ice (such as glare ice on a shaded bridge), CDL guidelines mandate slowing the commercial vehicle to a crawl (typically 5 to 15 mph5\text{ to }15\text{ mph}) or coming to a safe stop.
Ice drastically reduces friction, multiplying braking distance up to nine times compared to dry roads.
2
Identify the proper configuration for auxiliary braking devices on reduced-traction surfaces.
Turn off the engine retarder switch before traversing the slippery road section.
Retarder braking force is applied directly to the drive axle without anti-lock modulation on non-driven wheels, which can cause drive-wheel lockup and lead to a skid or jackknife.

Key Concept

Speed reduction guidelines for icy surfaces and auxiliary retarder safety rules
Estimated Time:1m 0s
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Topics

Speed and Road Conditions

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