A driver is operating a 70\text{-foot} commercial combination vehicle along an open multi-… — Commercial Driver's License (CDL) General Knowledge Exam

A driver is operating a $70\text{-foot}$ commercial combination vehicle along an open multi-lane highway at a steady cruising speed of $55\text{ mph}$ under dry, clear conditions. Using an overhead road sign as a stationary marker, the driver observes that the front bumper of the combination unit passes the marker $6.5\text{ seconds}$ after the rear of the lead vehicle cleared it. Based on standard commercial motor vehicle space management guidelines, which evaluation of this following distance and required driver action is correct?

A
The following distance is $0.5\text{ seconds}$ too short because a $70\text{-foot}$ vehicle requires a baseline of $7\text{ seconds}$ at any speed; the driver must ease off the accelerator to add half a second.
The following distance is $1.5\text{ seconds}$ too short because a $70\text{-foot}$ vehicle at $55\text{ mph}$ requires a minimum space cushion of $8\text{ seconds}$ ; the driver must increase following distance.Answer
C
The following distance is adequate because $6.5\text{ seconds}$ exceeds the basic total stopping distance and reaction-time threshold for tractor-trailer units on dry pavement.
D
The following distance is $1.0\text{ second}$ greater than necessary because heavy vehicles traveling above $50\text{ mph}$ require only $1\text{ second}$ per $15\text{ feet}$ of length plus $0.5\text{ seconds}$ ; the driver should close the gap.

Answer

The following distance is

1.5\text{ seconds}

too short because a

70\text{-foot}

vehicle operating at

55\text{ mph}

requires a minimum of

8\text{ seconds}

of following space, requiring the driver to drop back and increase space.

According to the official commercial driver rules for following distance, a driver must maintain at least

1\text{ second}

for each

10\text{ feet}

of vehicle length under

40\text{ mph}

, and add

1\text{ second}

when traveling at speeds of

40\text{ mph}

or greater. For a

70\text{-foot}

vehicle at

55\text{ mph}

, the required cushion is

7 + 1 = 8\text{ seconds}

. Since the driver measured only

6.5\text{ seconds}

, the vehicle is

1.5\text{ seconds}

too close and must increase spacing.

Step-by-Step Solution

Calculate the base following interval based on vehicle length.

\frac{70\text{ ft}}{10\text{ ft/s}} = 7\text{ seconds}

The commercial vehicle guideline provides

1\text{ second}

for every

10\text{ feet}

of vehicle length at speeds below

40\text{ mph}

Apply the speed-dependent modifier for high-speed operation.

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

When operating at or above

40\text{ mph}

, the driver must add

1\text{ extra second}

to the total following distance for safety.

Compare the measured time gap to the calculated requirement and determine driver response.

8.0\text{ s} - 6.5\text{ s} = 1.5\text{ seconds deficit}

The current interval of

6.5\text{ seconds}

falls

1.5\text{ seconds}

below the required minimum cushion of

8\text{ seconds}

, requiring the driver to increase spacing.

Key Concept

Calculation of commercial vehicle safe following distance using the vehicle length rule plus high-speed modifier.

Estimated Time:1m 30s

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