While operating an 80\text{-foot} multi-trailer combination unit at a steady speed of 55\t… — Commercial Driver's License (CDL) General Knowledge Exam

While operating an $80\text{-foot}$ multi-trailer combination unit at a steady speed of $55\text{ mph}$ on a dry, open highway, a driver uses an overhead signpost to track the vehicle ahead. The rear of the lead vehicle passes the signpost, and the commercial driver's front bumper reaches the same signpost exactly $7\text{ seconds}$ later. Based on standard commercial vehicle space management rules, how should the driver assess this following distance?

A
The following distance is insufficient; the vehicle requires a minimum safe gap of $8\text{ seconds}$ .
The following distance is insufficient; the vehicle requires a minimum safe gap of $9\text{ seconds}$ .Answer
C
The following distance is adequate; an $80\text{-foot}$ vehicle requires a baseline gap of only $7\text{ seconds}$ .
D
The following distance is adequate; commercial vehicles maintain a fixed $5\text{-second}$ buffer regardless of combination length.

Answer

The following distance is insufficient; the vehicle requires a minimum safe gap of

9\text{ seconds}

Under standard CDL space management guidelines, a driver must maintain at least

1\text{ second}

for every

10\text{ feet}

of vehicle length, plus an additional

1\text{ second}

when traveling at speeds of

40\text{ mph}

or greater. For an

80\text{-foot}

combination at

55\text{ mph}

, the required interval is

\frac{80}{10} + 1 = 9\text{ seconds}

. Because the driver counted only

7\text{ seconds}

between the lead vehicle and their own vehicle, the current gap is insufficient.

Step-by-Step Solution

Calculate the base following distance interval using vehicle length.

\frac{80\text{ feet}}{10\text{ feet/second}} = 8\text{ seconds}

Commercial driving standards specify a baseline of

1\text{ second}

for every

10\text{ feet}

of total vehicle length.

Apply the speed-based adjustment factor.

8\text{ seconds} + 1\text{ second} = 9\text{ seconds}

When operating at speeds of

40\text{ mph}

or higher, an additional

1\text{ second}

must be added to account for increased stopping distance.

Compare the actual measured time against the calculated minimum requirement.

The measured interval of

7\text{ seconds}

2\text{ seconds}

below the required minimum of

9\text{ seconds}

Because the measured time gap is less than the calculated threshold, the current following distance is unsafe.

Key Concept

Calculation and landmark verification of safe following distance based on vehicle length and highway operating speed.

Estimated Time:1m 30s

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