A commercial driver is operating a 65\text{-foot} tanker truck at a steady cruising speed … — Commercial Driver's License (CDL) General Knowledge Exam

A commercial driver is operating a $65\text{-foot}$ tanker truck at a steady cruising speed of $55\text{ mph}$ on a dry multi-lane highway under clear weather conditions. According to the commercial driver manual following distance formula, what is the minimum safe following distance the driver should maintain behind the vehicle ahead?

A
$6.5\text{ seconds}$
$7.5\text{ seconds}$ Answer
C
$5.5\text{ seconds}$
D
$4.0\text{ seconds}$

Answer

7.5\text{ seconds}

Under standard CDL safety guidelines, the following distance formula allows

1\text{ second}

per

10\text{ feet}

of vehicle length for speeds under

40\text{ mph}

. For a

65\text{-foot}

vehicle, the base allowance is

6.5\text{ seconds}

. Because the vehicle is traveling at

55\text{ mph}

(which is

\ge 40\text{ mph}

), an additional

1\text{ second}

must be added, yielding a required minimum following distance of

7.5\text{ seconds}

Step-by-Step Solution

Calculate the base following distance based on overall vehicle length.

Base time gap =

65\text{ ft} \div 10\text{ ft/s} = 6.5\text{ seconds}

The standard commercial driving rule requires at least

1\text{ second}

of space for each

10\text{ feet}

of vehicle length when driving under

40\text{ mph}

Evaluate operating speed and add the high-speed safety buffer if necessary.

Speed is

55\text{ mph}

(which is

\ge 40\text{ mph}

), requiring

+1.0\text{ second}

At speeds at or above

40\text{ mph}

, vehicles require significantly longer total stopping distances, necessitating an extra second of following gap.

Sum the base following interval and the high-speed adjustment.

6.5\text{ seconds} + 1.0\text{ second} = 7.5\text{ seconds}

Combining both components determines the absolute minimum safe time gap needed under ideal conditions.

Key Concept

Calculating minimum safe following distance using the 1 second per 10 feet rule plus 1 second for speeds at or over 40 mph

Estimated Time:1m 0s

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