A commercial driver operating a 55\text{-foot} tractor-semitrailer at 45\text{ mph} on wet… — Commercial Driver's License (CDL) General Knowledge Exam

A commercial driver operating a $55\text{-foot}$ tractor-semitrailer at $45\text{ mph}$ on wet pavement marks an overhead sign as the lead vehicle passes underneath and counts $6.5\text{ seconds}$ before their own front bumper reaches that sign. Under standard commercial driver licensing safety rules, this timed interval provides an adequate following distance for the prevailing operating conditions.

Answer: Answer

Answer

False, because while

6.5\text{ seconds}

satisfies the dry road formula, commercial drivers must increase following distance beyond the baseline when operating on wet pavement.

The statement is false because the calculated interval of

6.5\text{ seconds}

(

5.5\text{ seconds}

for length plus

1.0\text{ second}

for traveling at

45\text{ mph}

) represents only the minimum baseline for dry pavement. Operating on wet pavement reduces traction and increases stopping distance, requiring drivers to add additional space cushion beyond the baseline.

Step-by-Step Solution

Calculate the baseline following distance for dry pavement based on vehicle length.

55\text{-foot}

combination requires

5.5\text{ seconds}

(

1\text{ second}

for every

10\text{ feet}

of vehicle length).

The foundational CDL formula mandates one second of space cushion per ten feet of vehicle length.

Apply the speed-based modifier for speeds at or exceeding

40\text{ mph}

Add

1.0\text{ second}

, bringing the dry-pavement total to

5.5 + 1.0 = 6.5\text{ seconds}

At speeds of

40\text{ mph}

and above, higher momentum and braking distances require an additional second.

Adjust the required headway for environmental and surface conditions.

On wet pavement, the required safe following distance must exceed

6.5\text{ seconds}

Tire traction is reduced on wet surfaces, extending vehicle stopping distance and requiring a larger safety cushion.

Key Concept

CDL Following Distance Formula and Adverse Surface Adjustments

Estimated Time:1m 30s

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