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Difficulty: HardSpeed Control and Following Distance

Under ideal dry road conditions, defensive driving guidelines recommend maintaining at least a 33-second following distance behind a lead vehicle. However, while traveling on a highway with a posted speed limit of 65 mph65\text{ mph}, a driver encounters a sudden light rain that mixes with oil residue to make the asphalt slick, while simultaneously following directly behind a large commercial truck that obstructs the view ahead. Considering the combined effects of reduced traction and restricted forward visibility, what is the minimum following distance the driver should maintain?

  1. At least 66 seconds, expanding the time gap to compensate for both reduced stopping traction and restricted line-of-sight.Answer
  2. B
    Exactly 33 seconds, because the standard three-second rule is calculated to provide sufficient stopping space under all weather and vehicle conditions.
  3. C
    44 seconds, because adding a single extra second is sufficient whenever following a large truck, regardless of road surface slickness.
  4. D
    22 seconds, provided the driver maintains the posted speed limit of 65 mph65\text{ mph} to keep pace with surrounding traffic flow.

Answer

The driver should establish a minimum following distance of at least 66 seconds to account for both slippery road conditions and obstructed forward visibility.
Under standard dry conditions, a 33-second following distance provides adequate reaction time. However, safe driving principles require compounding the distance when multiple hazards exist. Following a large commercial truck obstructs forward vision, adding 11 to 22 seconds to the required gap. Additionally, light rain mixing with road oil produces a slick surface that reduces tire traction and increases stopping distance, adding another 11 to 22 seconds. Combining these factors requires expanding the safety buffer to at least 66 seconds.

Step-by-Step Solution

1
Identify the baseline following distance under ideal conditions.
The baseline safety gap on dry pavement for passenger vehicles is 33 to 44 seconds.
Establishes the foundation before adjusting for environmental and traffic hazards.
2
Account for the visibility hazard caused by the commercial truck.
Following a large vehicle restricts sightlines of hazards ahead, requiring an increase of at least 11 to 22 seconds to the gap (44 to 55 seconds total).
Extra space allows the driver to see around the truck and provides time to react if the truck stops suddenly.
3
Account for the surface condition hazard (light rain on oil residue).
Slick pavement significantly increases vehicle braking distances, requiring an additional 11 to 22 seconds of buffer space.
Reduced tire traction increases the physical distance required to bring the vehicle to a complete stop.
4
Combine the risk factors to calculate the total required following distance.
Adding the safety adjustments for both obstructed vision and reduced friction results in a minimum requirement of at least 66 seconds.
Multiple simultaneous hazards compound driving risk and necessitate cumulative safety buffers.

Key Concept

Cumulative Following Distance Adjustment for Multiple Hazards
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Topics

Speed Control and Following Distance

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