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Difficulty: Very hardSpace Ahead and Following Distance

A commercial driver is operating an 80-foot80\text{-foot} double-trailer combination on a dry, level interstate at a constant speed of 65 mph65\text{ mph}. At this speed, the vehicle travels approximately 95 feet per second95\text{ feet per second}. Under these ideal conditions, what is the absolute minimum physical following distance the driver must maintain behind the vehicle ahead according to the standard commercial vehicle space management formula?

  1. 855 feet855\text{ feet}Answer
  2. B
    760 feet760\text{ feet}
  3. C
    950 feet950\text{ feet}
  4. D
    475 feet475\text{ feet}

Answer

855 feet855\text{ feet}
The standard CDL space management rule requires 1 second of following distance for every 10 feet of vehicle length, plus an additional 1 second if traveling at 40 mph or greater. For an 80-foot vehicle, the base time is 8 seconds. Since 65 mph is greater than 40 mph, 1 extra second is added, totaling 9 seconds of required following time. To find the minimum physical distance in feet, multiply the required time by the vehicle's speed in feet per second: 9 seconds × 95 feet/second = 855 feet.

Step-by-Step Solution

1
Calculate the base following time required for the vehicle's length.
80 feet/10 feet per second=8 seconds80\text{ feet} / 10\text{ feet per second} = 8\text{ seconds}
The standard space management rule requires 1 second of following distance for every 10 feet of commercial vehicle length.
2
Determine if a speed adjustment is required.
Add 1 second1\text{ second} to the base time.
The vehicle is traveling at 65 mph65\text{ mph}, which is equal to or greater than the 40 mph40\text{ mph} threshold that requires an additional 1 second1\text{ second} of space.
3
Calculate the total required following time.
8 seconds+1 second=9 seconds8\text{ seconds} + 1\text{ second} = 9\text{ seconds}
Combining the length requirement and the high-speed safety buffer gives the total minimum time gap.
4
Convert the total required time into physical distance in feet.
9 seconds×95 feet per second=855 feet9\text{ seconds} \times 95\text{ feet per second} = 855\text{ feet}
The question asks for the physical space in feet, which is found by multiplying the required time gap by the vehicle's velocity in feet per second.

Key Concept

Calculating safe physical following distance by synthesizing the CDL time-based space management formula with velocity conversions.
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Topics

Space Ahead and Following Distance

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