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Difficulty: MediumSpace Ahead and Following Distance

During daytime travel on a dry arterial roadway, a freight driver steers a 90-foot90\text{-foot} multi-unit truck at a steady cruising speed of 38 mph38\text{ mph}. Under standard commercial vehicle spacing guidelines, what minimum time gap must be maintained between this truck and the vehicle directly ahead?

  1. A
    5 seconds5\text{ seconds}
  2. 9 seconds9\text{ seconds}Answer
  3. C
    10 seconds10\text{ seconds}
  4. D
    12 seconds12\text{ seconds}

Answer

9 seconds9\text{ seconds}
The correct answer is 9 seconds9\text{ seconds}. According to the standard CDL following distance formula, drivers must maintain at least 1 second for each 10 feet of vehicle length at speeds under 40 mph40\text{ mph}. For a 90-foot90\text{-foot} vehicle traveling at 38 mph38\text{ mph}, the calculation is 90/10=9 seconds90 / 10 = 9\text{ seconds}, with no additional speed penalty added.

Step-by-Step Solution

1
Determine the base time interval based on vehicle length
90 feet10 feet/second=9 seconds\frac{90\text{ feet}}{10\text{ feet/second}} = 9\text{ seconds}
Commercial vehicle safety standards require 1 second of following distance for every 10 feet of vehicle length.
2
Check whether the operating speed requires an additional speed modifier
The operating speed is 38 mph38\text{ mph}, which is below 40 mph40\text{ mph}; no additional second is added.
The rule mandates adding 1 extra second only for speeds at or above 40 mph40\text{ mph}.
3
Compute total required following distance
9 seconds+0 seconds=9 seconds9\text{ seconds} + 0\text{ seconds} = 9\text{ seconds}
Summing the length-based requirement with the speed modifier gives the total safe time buffer.

Key Concept

Commercial Vehicle Following Distance Rule (1 second per 10 feet below 40 mph)
Estimated Time:1m 0s
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Topics

Space Ahead and Following Distance

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