A commercial driver operating an 80-foot multi-trailer combination vehicle is traveling at… — Commercial Driver's License (CDL) General Knowledge Exam

A commercial driver operating an $80$ -foot multi-trailer combination vehicle is traveling at a cruising speed of $55\text{ mph}$ on a dry, level highway. Using a stationary overhead gantry as a visual reference point, the driver counts an elapsed following time of $5.5\text{ seconds}$ behind the vehicle ahead. According to standard commercial driver safety rules, by how many seconds must the driver increase the current following distance to meet the minimum required space cushion?

Answer: 3.5 seconds

Answer

3.5 seconds

According to the official CDL guidelines, the minimum following distance formula requires 1 second for every 10 feet of vehicle length under 40 mph, plus 1 additional second for speeds at or above 40 mph. For an

80

-foot combination vehicle traveling at

55\text{ mph}

, the base time is

80 / 10 = 8.0\text{ seconds}

, plus

1.0\text{ second}

for the speed threshold, totaling

9.0\text{ seconds}

. Because the driver currently maintains a

5.5\text{-second}

gap, the following distance must be increased by

9.0 - 5.5 = 3.5\text{ seconds}

Step-by-Step Solution

Calculate the base following distance required for the vehicle length

8.0 seconds

Standard commercial driving safety guidelines require at least 1 second for every 10 feet of vehicle length.

Apply the speed-based safety adjustment threshold

9.0 seconds total required space cushion

When operating at speeds of 40 mph or greater, exactly 1 additional second must be added to the vehicle length requirement.

Determine the shortfall between the required space cushion and the observed following distance

3.5 seconds

Subtracting the current 5.5-second following interval from the 9.0-second total requirement yields the additional time buffer required.

Key Concept

Application of the commercial vehicle following distance formula (1 second per 10 feet of length plus 1 second for speeds at or over 40 mph) and calculation of following distance deficit.

Estimated Time:1m 30s

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