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Difficulty: MediumSpace Ahead and Following Distance

A commercial driver is operating a 55-foot55\text{-foot} tractor-trailer combination at a steady speed of 45 mph45\text{ mph} on a clear, dry highway. According to the standard commercial motor vehicle following distance guidelines, what is the minimum safe following distance the driver should maintain behind the vehicle ahead?

  1. A
    5.5 seconds5.5\text{ seconds}
  2. 6.5 seconds6.5\text{ seconds}Answer
  3. C
    4.5 seconds4.5\text{ seconds}
  4. D
    7.5 seconds7.5\text{ seconds}

Answer

6.5 seconds6.5\text{ seconds}
The standard commercial driver space cushion rule specifies 1 second for every 10 feet of vehicle length at speeds under 40 mph, plus 1 additional second for speeds of 40 mph or above. For a 55-foot55\text{-foot} vehicle traveling at 45 mph45\text{ mph}, the base requirement is 5.5 seconds5.5\text{ seconds} plus 1 second1\text{ second} for speed, totaling 6.5 seconds6.5\text{ seconds}.

Step-by-Step Solution

1
Calculate the base following distance required for vehicle length under standard guidelines.
For a 55-foot55\text{-foot} vehicle: 5510=5.5 seconds\frac{55}{10} = 5.5\text{ seconds}.
The commercial vehicle standard requires 1 second of following distance for every 10 feet of vehicle length at speeds under 40 mph.
2
Evaluate the vehicle speed and apply the speed modifier.
Vehicle speed is 45 mph45\text{ mph}, which is ≥40 mph\geq 40\text{ mph}, requiring an additional 1 second1\text{ second}.
At speeds of 40 mph or greater, drivers must add 1 extra second of following distance to account for increased braking and stopping distances.
3
Sum the base time and the speed adjustment to find the total minimum following interval.
5.5 seconds+1.0 second=6.5 seconds5.5\text{ seconds} + 1.0\text{ second} = 6.5\text{ seconds}.
Combining the base length requirement and speed modifier yields the total safe following gap under dry, ideal conditions.

Key Concept

Commercial vehicle following distance calculation using the 1-second-per-10-feet formula plus 1 second for speeds at or over 40 mph.
Estimated Time:1m 0s
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Topics

Space Ahead and Following Distance

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