While operating a commercial vehicle on a highway with a posted dry-road speed limit of 48… — Commercial Driver's License (CDL) General Knowledge Exam

While operating a commercial vehicle on a highway with a posted dry-road speed limit of $48\text{ mph}$ , a driver encounters persistent rainfall that creates wet pavement with standing water. Which combination of speed adjustment and vehicle control action is required under these road conditions?

A
Maintain the posted speed of $48\text{ mph}$ and increase the engine retarder setting to provide automatic braking assistance.
Reduce speed to at least $32\text{ mph}$ and switch off the engine retarder.Answer
C
Maintain cruising speed on straight sections and brake firmly while steering through curves to scrub off excess momentum.
D
Reduce speed by only $5\text{ mph}$ , assuming anti-lock braking systems eliminate the need for additional perception and reaction distances on wet roads.

Answer

Reduce speed by at least one-third (to

32\text{ mph}

or lower) and disable the engine retarder.

Commercial driving regulations dictate that drivers must reduce their speed by at least one-third on wet roads (reducing

48\text{ mph}

32\text{ mph}

) to maintain steering control and prevent hydroplaning. Additionally, engine retarders must be turned off whenever driving on wet, snowy, or icy surfaces because their braking torque can break drive-wheel traction and cause an unrecoverable skid.

Step-by-Step Solution

Calculate the mandatory speed reduction for wet pavement conditions.

On wet roads, commercial vehicle speed must be reduced by at least one-third:

48\text{ mph} - \left(\frac{1}{3} \times 48\text{ mph}\right) = 48\text{ mph} - 16\text{ mph} = 32\text{ mph}

Wet surfaces reduce tire traction, requiring lower speeds to prevent hydroplaning and manage longer stopping distances.

Adjust auxiliary braking devices according to road surface friction.

Switch the engine retarder to the OFF position.

When drive wheels have reduced traction on wet pavement, retarder braking force applies solely to drive axles, which can cause them to lock up and initiate an immediate skid.

Key Concept

Speed reduction on wet pavement and engine retarder safety

Estimated Time:1m 15s

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