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Question

Difficulty: MediumNight Driving

A commercial motor vehicle driver travels along a dark rural corridor where low-beam headlights illuminate objects up to 250 feet ahead. To prevent overdriving the headlights, which operational standard must determine the vehicle's travel speed?

  1. A
    The travel speed only needs to ensure that the mechanical braking distance alone does not exceed the illuminated sight range.
  2. The travel speed must ensure total stopping distance, including perception, reaction, and braking, remains within the distance illuminated by the headlights.Answer
  3. C
    The travel speed can remain at the daytime posted limit provided the driver maintains a standard four-second following interval behind preceding vehicles.
  4. D
    The travel speed may exceed the headlight illumination limit on straight sections because high beams can be switched on immediately when an obstruction appears.

Answer

The travel speed must ensure total stopping distance, including perception, reaction, and braking, remains within the distance illuminated by the headlights.
Safe night driving requires that a commercial vehicle's total stopping distance (perception distance + reaction distance + brake lag distance + braking distance) never exceeds the forward distance illuminated by its headlights. This guarantees the driver can identify an unexpected roadway hazard and come to a complete stop before reaching it.

Step-by-Step Solution

1
Identify the primary constraint governing nighttime driving safety.
Headlights provide limited forward visibility (typically 250 feet on low beam and 350 to 500 feet on high beam).
Visibility is substantially restricted at night compared to daytime conditions.
2
Evaluate the definition and danger of overdriving headlights.
Overdriving headlights occurs when total stopping distance exceeds the visible illuminated distance ahead.
If a hazard appears at the outer edge of illumination, the vehicle cannot stop in time unless total stopping distance fits inside that range.
3
Account for all components of total stopping distance.
Total stopping distance is the sum of perception distance, reaction distance, brake lag distance, and effective braking distance.
Drivers require time to recognize a hazard and apply the brakes before mechanical deceleration even begins.

Key Concept

Headlight sight distance and total stopping distance management at night
Estimated Time:1m 0s
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Topics

Night Driving

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