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Question

Difficulty: HardNight Driving

A commercial driver is operating a loaded combination vehicle on an unlit secondary highway where continuous oncoming traffic restricts illumination to low-beam headlights, providing forward sight distance of approximately 250 feet. At a cruising speed of 55 mph, the vehicle's total stopping distance—including perception, reaction, air brake lag, and effective braking—is roughly 300 to 400 feet. To avoid overdriving the headlights and prevent collisions with unexpected road hazards, which operational action should the driver take?

  1. A
    Keep travel speed unchanged while increasing following distance behind lead vehicles to six seconds, relying on forward tail lights to identify road obstacles.
  2. Lower travel speed until the combined distance required for perception, reaction, air brake lag, and physical braking falls completely within the forward distance illuminated by the low beams.Answer
  3. C
    Maintain the posted speed limit and depend on aggressive air brake application to shorten the mechanical braking distance if a stationary hazard enters view.
  4. D
    Sustain current highway speed while scanning past the illuminated zone toward the dark horizon to predict upcoming road path changes.

Answer

Lower travel speed until the combined distance required for perception, reaction, air brake lag, and physical braking falls completely within the forward distance illuminated by the low beams.
Under night driving conditions, a commercial motor vehicle must be operated at a speed that allows it to stop completely within the distance illuminated by its headlights. When low beams restrict visibility to roughly 250 feet, driving at 55 mph requires a stopping distance (300–400 feet) that exceeds headlight range. The only safe corrective action is to reduce speed so that perception, reaction, brake lag, and braking distances all fit within the 250-foot sight boundary.

Step-by-Step Solution

1
Analyze the sight distance constraint under low-beam headlight operation.
Identified that low beams provide approximately 250 feet of clear forward visibility on an unlit road.
At night, a commercial driver can only react to and avoid obstacles that are visible within the illuminated headlight beam.
2
Evaluate total stopping distance components against visible sight distance.
Determined that at 55 mph, the combination of perception distance, reaction distance, air brake lag, and braking distance (300–400 feet) exceeds the 250-foot sight envelope.
Traveling at a speed where stopping distance exceeds sight distance creates the hazard of overdriving headlights, meaning a driver cannot stop before striking an unlit object.
3
Select the proper operational countermeasure to maintain safe control.
The driver must decelerate to a speed where total stopping distance is strictly less than or equal to the 250 feet illuminated by the low beams.
Speed reduction directly shortens perception-reaction distance and braking distance, ensuring the vehicle can come to a full stop within the visible path.

Key Concept

Speed management to prevent overdriving headlights during night operations
Estimated Time:1m 30s
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Topics

Night Driving

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