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Difficulty: Very hardSpeed and Road Conditions

You are operating a loaded commercial combination vehicle at 60 mph60\text{ mph} on a highway when heavy rain begins to fall, causing water to pool in the ruts of the road. You are approaching an off-ramp curve that has a posted dry-weather advisory speed of 45 mph45\text{ mph}. Your engine retarder is currently active. Which of the following is the strictly correct sequence of actions to safely navigate the approaching curve under these conditions?

  1. Deactivate the engine retarder immediately to prevent drive-wheel lockup, and begin decelerating early to reach a maximum speed of 30 mph30\text{ mph} before entering the curve.Answer
  2. B
    Keep the engine retarder engaged on a low setting to avoid using the service brakes, allowing the engine to safely slow the vehicle down to 45 mph45\text{ mph} before entering the curve.
  3. C
    Deactivate the engine retarder, coast into the curve at your current speed, and then apply steady pressure to the service brakes while steering to reduce your speed to 30 mph30\text{ mph}.
  4. D
    Deactivate the engine retarder and calculate your deceleration point based solely on the vehicle's braking distance, applying the brakes at your normal dry-road marker to reach 30 mph30\text{ mph} before the curve.

Answer

Deactivate the engine retarder immediately to prevent drive-wheel lockup, and begin decelerating early to reach a maximum speed of 30 mph before entering the curve.
The correct sequence safely addresses all hazards present. First, turning off the engine retarder is mandatory on wet roads to prevent drive-wheel lockup. Second, the CDL standard dictates reducing speed by one-third on wet roads, making 30 mph the correct target for a 45 mph advisory curve. Finally, all braking must be completed before entering the curve to ensure all available traction is reserved for steering.

Step-by-Step Solution

1
Identify the immediate hazard regarding the vehicle's equipment in wet conditions.
The engine retarder must be turned off immediately because using it on wet, slippery roads can cause the drive wheels to lock up and cause a skid.
Retarders apply braking force only to the drive wheels, which easily lose traction on wet surfaces.
2
Calculate the safe speed for the upcoming curve based on the road conditions.
The dry advisory speed is 45 mph. The CDL manual requires reducing speed by about one-third on wet roads. One-third of 45 is 15. Therefore, the safe speed is 45 - 15 = 30 mph.
Wet roads reduce traction, meaning standard posted and advisory speeds are no longer safe.
3
Determine the correct timing and location for the speed reduction.
The driver must begin decelerating earlier than normal due to increased stopping distances and must reach the target speed of 30 mph before entering the curve.
Braking while turning in a curve increases the risk of lateral skidding, and wet roads require longer total stopping distances.

Key Concept

Synthesizing speed management on wet roads, proper curve negotiation, and engine retarder restrictions.
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Topics

Speed and Road Conditions

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