A commercial driver is operating an 80\text{-foot} heavy combination vehicle on a dry inte… — Commercial Driver's License (CDL) General Knowledge Exam

A commercial driver is operating an $80\text{-foot}$ heavy combination vehicle on a dry interstate at a constant speed of $60\text{ mph}$ (which translates to $88\text{ feet per second}$ ). A passenger vehicle abruptly changes lanes directly in front of the truck, leaving a physical gap of $308\text{ feet}$ between the two vehicles.

Assuming the commercial vehicle maintains its speed, what is the truck's current time gap to the passenger vehicle, and exactly how much additional following distance must the driver establish to comply with the standard CDL minimum safe space ahead formula?

Current time gap: $3.5\text{ seconds}$ ; Additional distance required: $484\text{ feet}$ Answer
B
Current time gap: $3.5\text{ seconds}$ ; Additional distance required: $396\text{ feet}$
C
Current time gap: $5.1\text{ seconds}$ ; Additional distance required: $234\text{ feet}$
D
Current time gap: $3.5\text{ seconds}$ ; Additional distance required: $616\text{ feet}$

Answer

The correct answer is a current time gap of

3.5\text{ seconds}

and an additional distance requirement of

484\text{ feet}

The correct answer accurately determines both parameters. First, the current time gap is found by dividing the physical gap (

308\text{ feet}

) by the speed (

88\text{ feet per second}

), yielding

3.5\text{ seconds}

. Next, the standard CDL formula requires

1\text{ second}

of following distance for every

10\text{ feet}

of vehicle length. For an

80\text{-foot}

vehicle, this is

8\text{ seconds}

. Because the speed is at or above

40\text{ mph}

1\text{ additional second}

must be added, totaling a required space of

9\text{ seconds}

. At

88\text{ ft/s}

, a

9\text{-second}

gap equals

792\text{ feet}

. The additional distance needed is the difference between the required distance and the current physical gap (

792\text{ feet} - 308\text{ feet} = 484\text{ feet}

Step-by-Step Solution

Calculate the current time gap.

Divide the physical gap (

308\text{ feet}

) by the speed (

88\text{ ft/s}

) to get

3.5\text{ seconds}

This establishes the exact reaction time window the driver currently has.

Determine the base required following time.

Divide the vehicle length (

80\text{ feet}

) by

10

to get

8\text{ seconds}

The standard CDL formula requires

1\text{ second}

of following distance for every

10\text{ feet}

of vehicle length.

Apply the speed modifier.

Add

1\text{ second}

because the speed (

60\text{ mph}

) is at or above

40\text{ mph}

, totaling

9\text{ seconds}

Higher speeds demand a greater space cushion to allow for perception and reaction time.

Calculate the required physical distance.

Multiply the required time (

9\text{ seconds}

) by the speed (

88\text{ ft/s}

) to get

792\text{ feet}

This converts the required time gap into a measurable physical distance on the roadway.

Calculate the distance deficit.

Subtract the current gap (

308\text{ feet}

) from the required distance (

792\text{ feet}

) to get

484\text{ feet}

This represents the exact distance the truck must drop back to re-establish a safe and compliant space cushion.

Key Concept

Calculating following distance deficits by comparing current time gaps to CDL length and speed requirements.

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