You are operating a 30-foot single-axle box truck at a steady cruising speed of 42\text{ m… — Commercial Driver's License (CDL) General Knowledge Exam

You are operating a $30$ -foot single-axle box truck at a steady cruising speed of $42\text{ mph}$ along a dry, two-lane rural road under daylight conditions. According to standard commercial vehicle safety guidelines, what is the minimum safe following distance you must maintain behind the vehicle ahead?

A
$2\text{ seconds}$
B
$3\text{ seconds}$
$4\text{ seconds}$ Answer
D
$6\text{ seconds}$

Answer

A minimum safe following distance of

4\text{ seconds}

The standard rule for commercial vehicles provides that a driver should maintain at least

1\text{ second}

for each

10\text{ feet}

of vehicle length at speeds below

40\text{ mph}

, and add

1\text{ second}

for speeds at or above

40\text{ mph}

. A

30

-foot vehicle requires

3\text{ seconds}

for length, plus

1\text{ second}

because speed is

42\text{ mph}

, totaling

4\text{ seconds}

Step-by-Step Solution

Calculate the base following distance based on vehicle length

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

The commercial vehicle standard requires

1\text{ second}

of following distance for each

10\text{ feet}

of vehicle length at speeds under

40\text{ mph}

Apply the speed adjustment modifier

Add

1\text{ second}

Because the vehicle is traveling at

42\text{ mph}

(which is

\geq 40\text{ mph}

), an additional second must be added to provide an adequate safety cushion.

Sum the base following distance and speed adjustment

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

Total required space ahead equals the length-based interval plus the high-speed interval.

Key Concept

CDL Space Ahead Formula: 1 second per 10 feet of vehicle length, plus 1 second for speeds at or above 40 mph

Estimated Time:1m 0s

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