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Difficulty: HardSpace Ahead and Following Distance

A commercial driver is operating a 55-foot55\text{-foot} refrigerated tractor-trailer along a dry, multi-lane divided highway at a constant speed of 45 mph45\text{ mph}. According to the standard CDL following distance formula, what is the minimum time interval the driver must maintain behind the lead vehicle?

  1. A
    5.5 seconds5.5\text{ seconds}
  2. 6.5 seconds6.5\text{ seconds}Answer
  3. C
    4.0 seconds4.0\text{ seconds}
  4. D
    7.5 seconds7.5\text{ seconds}

Answer

A minimum time interval of 6.5 seconds6.5\text{ seconds} must be maintained.
Under CDL guidelines, safe following distance is calculated as 1 second for every 10 feet of vehicle length when traveling under 40 mph, with an extra 1 second added for speeds of 40 mph and above. For a 55-foot55\text{-foot} combination at 45 mph45\text{ mph}, the required cushion is 5510+1=6.5 seconds\frac{55}{10} + 1 = 6.5\text{ seconds}.

Step-by-Step Solution

1
Calculate the base following interval based on vehicle length.
55 feet10 feet/second=5.5 seconds\frac{55\text{ feet}}{10\text{ feet/second}} = 5.5\text{ seconds}
The standard rule specifies that a commercial driver must allow at least 1 second for each 10 feet of vehicle length.
2
Evaluate the vehicle speed against the regulatory speed threshold.
Speed is 45 mph45\text{ mph}, which meets or exceeds the 40 mph40\text{ mph} threshold, requiring an additional +1.0 second+1.0\text{ second}.
For vehicle speeds at or above 40 mph, higher stopping momentum necessitates adding 1 extra second of space cushion.
3
Compute the total minimum following distance in seconds.
5.5 seconds+1.0 second=6.5 seconds5.5\text{ seconds} + 1.0\text{ second} = 6.5\text{ seconds}
Summing the length-based gap and the high-speed safety buffer yields the total required headway.

Key Concept

CDL Standard Following Distance Formula
Estimated Time:1m 0s
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Topics

Space Ahead and Following Distance

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