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Difficulty: MediumSpeed Limits and Speed Regulation

A driver is traveling on an unlit rural road at night where the posted speed limit is 50 mph50\text{ mph}. Heavy fog begins to roll in, reducing the driver's forward visibility to approximately 100 feet100\text{ feet}. Under the Basic Speed Law, what is the driver legally required to do?

  1. A
    Maintain 50 mph50\text{ mph} as long as the vehicle remains within the marked lane lines.
  2. Reduce speed to a rate that allows the vehicle to come to a complete stop within the visible distance ahead.Answer
  3. C
    Increase speed slightly to exit the fog zone as quickly as possible.
  4. D
    Maintain a constant speed of 45 mph45\text{ mph} because posted speed limits allow a 5 mph5\text{ mph} tolerance in adverse weather.

Answer

The driver must reduce speed to a rate that allows the vehicle to stop safely within the distance visible ahead.
Under the Basic Speed Law, drivers must adjust their speed to what is safe for current road and weather conditions. When visibility is severely impaired by fog, the driver must reduce speed so that total stopping distance does not exceed the illuminated or visible distance ahead.

Step-by-Step Solution

1
Identify the environmental hazard and its impact on driving safety.
Heavy fog reduces forward visibility to 100 feet100\text{ feet}, significantly limiting the driver's reaction time and visual buffer.
At 50 mph50\text{ mph}, total stopping distance (reaction distance plus braking distance) typically exceeds 200 feet200\text{ feet}, which is far greater than the 100 feet100\text{ feet} of visibility.
2
Apply the statutory Basic Speed Law to the scenario.
The law mandates that no person shall drive a vehicle at a speed greater than is reasonable or prudent under existing conditions.
Posted speed limits represent maximum allowable speeds under ideal conditions, not guaranteed safe speeds under adverse weather or reduced visibility.

Key Concept

Basic Speed Law in reduced visibility conditions
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Topics

Speed Limits and Speed Regulation

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