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Difficulty: HardNight Driving

A commercial motor vehicle is traveling at 55 mph on an unlighted highway where low-beam headlights illuminate approximately 250 feet ahead. At 55 mph under ideal conditions, the vehicle's total stopping distance—accounting for perception distance, reaction distance, air brake lag, and braking distance—is approximately 419 feet. Which statement correctly explains why operating at this speed with low beams creates an extreme safety hazard?

  1. The driver is overdriving the headlights, meaning the total distance required to come to a complete stop exceeds the illuminated sight distance, making a collision inevitable if an unlit hazard appears.Answer
  2. B
    The operation remains safe because the mechanical braking distance alone is less than 250 feet once the air brakes engage, allowing a skilled driver to stop before impact.
  3. C
    The hazard can be safely eliminated without reducing speed by looking toward the right edge of the travel lane to extend perceived sight distance beyond 419 feet.
  4. D
    The hazard exists only when following another vehicle within the standard following interval of one second per 10 feet of vehicle length.

Answer

The driver is overdriving the headlights because total stopping distance (419 feet) exceeds the distance illuminated by low beams (250 feet), preventing the vehicle from stopping within visible road distance.
At 55 mph, a commercial vehicle requires approximately 419 feet to come to a complete stop when accounting for perception, reaction, brake lag, and braking distances. Because low beams illuminate only about 250 feet ahead, the vehicle cannot stop within the illuminated area, meaning any unlit object on the roadway will be struck before the vehicle can halt.

Step-by-Step Solution

1
Analyze the vehicle's total stopping distance components at 55 mph.
Total stopping distance equals perception distance (~142 ft) + reaction distance (~61 ft) + air brake lag (~32 ft) + effective braking distance (~184 ft) = approximately 419 ft.
Safe vehicle operation requires accounting for all stages of deceleration, not just mechanical brake application.
2
Compare total stopping distance with the illumination sight distance of low-beam headlights.
Headlight sight distance (250 ft) is 169 ft shorter than total stopping distance (419 ft).
To avoid a stationary road hazard at night, a driver must be able to stop within the distance they can see ahead.
3
Determine the operational risk and correct safety principle.
Continuing at 55 mph constitutes overdriving the headlights; speed must be reduced so total stopping distance does not exceed 250 ft.
Commercial drivers must adjust travel speed to match their sight distance under nighttime driving conditions.

Key Concept

Overdriving Headlights and Night Total Stopping Distance
Estimated Time:1m 30s
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Topics

Night Driving

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