While operating a 30\text{-foot} single-unit delivery truck at a steady speed of 42\text{ … — Commercial Driver's License (CDL) General Knowledge Exam

While operating a $30\text{-foot}$ single-unit delivery truck at a steady speed of $42\text{ mph}$ on a clear, dry multi-lane highway, what is the minimum safe following distance you must maintain behind the vehicle ahead?

A
$3\text{ seconds}$
$4\text{ seconds}$ Answer
C
$2\text{ seconds}$
D
$5\text{ seconds}$

Answer

A minimum following distance of

4\text{ seconds}

is required.

Under standard commercial driving rules, a driver must allow at least

1\text{ second}

for each

10\text{ feet}

of vehicle length when driving under

40\text{ mph}

. For a

30\text{-foot}

truck, this equals

3\text{ seconds}

. Because the vehicle is traveling at

42\text{ mph}

(which is above

40\text{ mph}

), an additional

1\text{ second}

must be added, resulting in a total required following distance of

4\text{ seconds}

Step-by-Step Solution

Calculate the base following distance required for vehicle length.

30\text{ ft} \div 10\text{ ft/sec} = 3\text{ seconds}

CDL guidelines require

1\text{ second}

of following distance for each

10\text{ feet}

of vehicle length at speeds under

40\text{ mph}

Check the operating speed and apply the speed modifier if necessary.

42\text{ mph} \ge 40\text{ mph} \implies +1\text{ second}

When operating at or above

40\text{ mph}

, an additional

1\text{ second}

must be added to the total following distance.

Sum the base following distance and the speed adjustment.

3\text{ seconds} + 1\text{ second} = 4\text{ seconds}

Combining the base length requirement with the speed modifier gives the total minimum following buffer under ideal driving conditions.

Key Concept

CDL Following Distance Formula

Estimated Time:1m 0s

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