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Difficulty: MediumDriving in Fog

A commercial motor vehicle encounters an unexpected blanket of thick fog along a rural two-lane highway, causing moisture accumulation on the road surface and severely limiting visibility. What is the safest operational procedure for the driver under these specific conditions?

  1. A
    Activate high-beam headlights to cut through the water droplets and stay immediately behind a lead vehicle's taillights for guidance.
  2. B
    Engage the auxiliary engine retarder to its highest setting to assist braking on the slick surface while maintaining posted highway speeds.
  3. Switch to low-beam headlights, turn off the auxiliary engine retarder, and adjust travel speed so the vehicle can come to a full stop within the visible sight distance.Answer
  4. D
    Maintain normal road speed while preparing to apply hard service braking, assuming that total stopping distance is determined purely by brake friction force.

Answer

Switch to low-beam headlights, turn off the auxiliary engine retarder, and adjust travel speed so the vehicle can come to a full stop within the visible sight distance.
The correct answer properly combines the three core FMCSA defensive driving principles for fog: using low-beam headlights to minimize glare, disengaging auxiliary retarders to prevent skidding on moisture-slicked roads, and moderating speed so the vehicle can stop within the visible space ahead.

Step-by-Step Solution

1
Select the appropriate headlight setting for low-visibility fog conditions.
Low-beam headlights are selected to prevent high-beam reflection and glare off atmospheric water droplets.
High beams reflect light directly back into the driver's eyes, further degrading forward sight.
2
Assess the pavement condition resulting from fog condensation and adjust powertrain controls.
Auxiliary engine retarders are deactivated.
Moisture from fog creates a slick film on the asphalt, and active retarders can break drive-wheel traction and cause skids.
3
Establish a safe operating speed relative to forward visibility.
Vehicle speed is reduced so that total stopping distance is strictly within the clearly illuminated and visible forward space.
Federal safety standards require commercial drivers to always be capable of stopping within the distance they can see ahead.

Key Concept

Safe operating practices in fog require low-beam lighting, auxiliary retarder disengagement due to slippery condensation, and speed calibrated to sight distance.
Estimated Time:1m 0s
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Topics

Driving in Fog

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