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Difficulty: Very hardSpace Ahead and Following Distance

You are operating a 6060-foot combination vehicle on a dry interstate at 5555 mph (traveling approximately 8181 feet per second). You count the time between when the vehicle ahead passes a fixed landmark and when your front bumper reaches the same landmark, measuring a 44-second following distance.

To establish the minimum safe following distance required by the commercial driver standard formula, approximately how many additional feet of space must you create?

  1. A
    162 feet
  2. B
    567 feet
  3. 243 feetAnswer
  4. D
    486 feet

Answer

243 feet
According to the CDL heavy vehicle formula, a 6060-foot vehicle requires 66 seconds of base following distance (11 second per 1010 feet). Because the speed is 5555 mph (which is greater than the 4040 mph threshold), an additional 11 second is required, bringing the total required following distance to 77 seconds. The driver currently has a 44-second gap, leaving a deficit of 33 seconds. Traveling at 8181 feet per second, 33 seconds of time corresponds to 243243 feet of additional space required.

Step-by-Step Solution

1
Calculate the base following distance in seconds for the vehicle's length.
60÷10=660 \div 10 = 6 seconds.
The standard CDL heavy vehicle formula requires 1 second of space for every 10 feet of vehicle length.
2
Apply the speed adjustment to find the total required following time.
6+1=76 + 1 = 7 seconds total required time.
The vehicle is traveling at 5555 mph, and the formula strictly dictates adding 1 additional second for any speed of 4040 mph or greater.
3
Determine the time deficit.
7−4=37 - 4 = 3 seconds.
Subtracting the currently established following distance (44 seconds) from the required distance (77 seconds) identifies the missing safety buffer.
4
Convert the time deficit into physical distance.
3×81=2433 \times 81 = 243 feet.
Multiplying the time deficit by the vehicle's speed in feet per second translates the time gap into the exact distance the driver needs to fall back.

Key Concept

Space Ahead and Following Distance Formulas
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Topics

Space Ahead and Following Distance

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