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Difficulty: HardSpeed and Road Conditions

While hauling a loaded trailer along a divided highway at 50 mph50\text{ mph}, the ambient temperature drops below freezing and freezing drizzle begins coating elevated roadway surfaces with clear ice. Which combination of driver adjustments represents the correct procedure for safely managing vehicle speed and traction before reaching an upcoming overpass?

  1. A
    Reduce vehicle speed by one-third of the posted limit and select the highest engine retarder setting to conserve service brake friction on the slick bridge surface.
  2. Switch off the auxiliary retarder system and reduce road speed to a crawl well before entering the icy overpass structure.Answer
  3. C
    Maintain normal cruising speed until the front tires reach the bridge deck, then apply firm, continuous service braking while crossing the structure.
  4. D
    Reduce vehicle speed by one-half and maintain standard following distance, relying on the brake system to keep total stopping distance comparable to dry asphalt.

Answer

Switch off the auxiliary retarder system and reduce road speed to a crawl well before entering the icy overpass structure.
On icy road surfaces, standard commercial motor vehicle safety procedures mandate reducing speed to a crawl and switching off all auxiliary engine retarders to prevent drive-wheel skids. All speed adjustments must be completed prior to entering the hazard rather than braking while on the slippery structure.

Step-by-Step Solution

1
Identify the prevailing road surface condition and apply the corresponding commercial vehicle speed reduction guideline.
Freezing drizzle forms ice on exposed overpasses, requiring drivers to reduce speed to a crawl rather than standard wet (one-third) or packed snow (one-half) margins.
Ice drastically reduces the coefficient of friction between tires and pavement, requiring extremely low speeds to prevent uncontrolled skidding.
2
Evaluate the status of auxiliary braking devices (engine retarders) in slick conditions.
The auxiliary retarder must be switched off.
Retarders apply braking force exclusively to drive wheels; on slippery surfaces, this drag can cause drive wheels to lose traction instantly and trigger a jackknife.
3
Determine the timing of the speed adjustment relative to the road hazard.
Speed must be adjusted before reaching the icy overpass, avoiding active braking on the bridge deck.
Braking while transitioning onto or traversing a slick surface induces lateral instability and wheel slip.

Key Concept

Speed reduction to a crawl on icy surfaces, proactive deceleration before hazards, and disabling engine retarders in poor traction conditions.
Estimated Time:1m 30s
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Topics

Speed and Road Conditions

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