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Difficulty: Very hardSpace Ahead and Following Distance

During a safety audit, a fleet manager reviews telemetry data for a driver operating a 5050-foot commercial motor vehicle. The data reveals that the driver maintained a constant physical gap of exactly 450450 feet from the vehicle ahead during two distinct phases of a trip:

- Phase 1: Traveling in dense morning traffic at a steady 3030 mph (approximately 4444 feet per second).
- Phase 2: Traveling on an open highway at a steady 6060 mph (approximately 8888 feet per second).

Based on the standard commercial vehicle space management formulas, which of the following accurately evaluates the compliance of the 450450-foot gap during both phases?

  1. The gap is compliant during Phase 1 (which requires 220220 feet), but non-compliant during Phase 2 because speeds of 4040 mph or greater demand an additional second of time, bringing the minimum required gap to 528528 feet.Answer
  2. B
    The gap is compliant during both phases, because the standard formula dictates a strict 55 seconds of following distance for a 5050-foot vehicle regardless of speed, which equates to only 440440 feet at 6060 mph.
  3. C
    The gap is compliant during Phase 1 but non-compliant during Phase 2 because guidelines require 11 second of following distance for every 1010 mph of travel speed, completely independent of the vehicle's physical length.
  4. D
    The gap is non-compliant during Phase 2 because following distance rules strictly mandate maintaining a physical gap equal to the vehicle's total stopping distance, which accounts for perception and reaction times.

Answer

The gap is compliant during Phase 1 (which requires 220220 feet), but non-compliant during Phase 2 because speeds of 4040 mph or greater demand an additional second of time, bringing the minimum required gap to 528528 feet.
The CDL manual instructs drivers to leave 11 second of following distance for every 1010 feet of vehicle length at speeds under 4040 mph, and to add 11 extra second for speeds of 4040 mph or higher. For a 5050-foot vehicle at 3030 mph, the requirement is 55 seconds (5×44=2205 \times 44 = 220 feet), making the 450450-foot gap safe. At 6060 mph, the requirement increases to 66 seconds (6×88=5286 \times 88 = 528 feet), meaning the 450450-foot gap is suddenly deficient by 7878 feet.

Step-by-Step Solution

1
Calculate the base time required for a 5050-foot vehicle.
5050 feet / 1010 = 55 seconds base time.
The standard rule mandates 11 second of space for every 1010 feet of vehicle length.
2
Evaluate Phase 1 compliance (3030 mph).
Speed is under 4040 mph, so the requirement remains 55 seconds. Physical distance required is 5 seconds×44 fps=220 feet5 \text{ seconds} \times 44 \text{ fps} = 220 \text{ feet}.
Since the 450450-foot actual gap is greater than 220220 feet, the vehicle is fully compliant.
3
Evaluate Phase 2 compliance (6060 mph).
Speed is 4040 mph or greater, so add 11 second (5+1=65 + 1 = 6 seconds). Physical distance required is 6 seconds×88 fps=528 feet6 \text{ seconds} \times 88 \text{ fps} = 528 \text{ feet}.
Since the 450450-foot actual gap is less than the required 528528 feet, the vehicle is non-compliant.

Key Concept

Commercial vehicle following distance calculation is based on vehicle length, with a critical time adjustment threshold triggered at speeds of 4040 mph or higher.

Alternative Method

Instead of converting the required time into feet, you can convert the physical 450450-foot gap into a time measurement to compare against the time requirements. Phase 1: 450÷44≈10.2450 \div 44 \approx 10.2 seconds (which easily exceeds the 55-second requirement). Phase 2: 450÷88≈5.1450 \div 88 \approx 5.1 seconds (which falls short of the 66-second requirement).
Estimated Time:2m 0s
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Topics

Space Ahead and Following Distance

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