A commercial motor vehicle equipped with an air brake system is traveling at 55\text{ mph}… — Commercial Driver's License (CDL) General Knowledge Exam

A commercial motor vehicle equipped with an air brake system is traveling at $55\text{ mph}$ on a dry, level roadway. Under standard CDL testing conditions, the total stopping distance from initial hazard recognition to a complete stop is $451\text{ feet}$ . This total includes a perception distance of $142\text{ feet}$ , a reaction distance of $61\text{ feet}$ , and an air brake lag distance of $32\text{ feet}$ . What is the vehicle's actual braking distance (the distance traveled after the brakes physically engage until the vehicle stops), in feet?

Answer: 216 feet

Answer

The actual braking distance is

216\text{ feet}

For an air brake-equipped commercial vehicle, Total Stopping Distance is the sum of Perception Distance (

142\text{ ft}

), Reaction Distance (

61\text{ ft}

), Brake Lag Distance (

32\text{ ft}

), and Braking Distance. Subtracting the first three components (

235\text{ ft}

) from the total stopping distance of

451\text{ feet}

yields an actual braking distance of

216\text{ feet}

Step-by-Step Solution

Express the total stopping distance formula for vehicles equipped with air brakes.

\text{Total Stopping Distance} = \text{Perception Distance} + \text{Reaction Distance} + \text{Brake Lag Distance} + \text{Braking Distance}

Air brake systems have four distinct stopping distance components due to the mechanical delay of air moving through the brake lines.

Calculate the combined distance traveled during the pre-braking phases (perception, reaction, and air brake lag).

142\text{ ft} + 61\text{ ft} + 32\text{ ft} = 235\text{ ft}

These phases occur before the brake linings physically create friction against the drums or discs to begin decelerating the vehicle.

Subtract the pre-braking distance from the total stopping distance to determine the actual braking distance.

451\text{ ft} - 235\text{ ft} = 216\text{ ft}

Subtracting the non-braking distance from the overall stopping distance isolates the distance traveled while active friction braking occurs.

Key Concept

Four-component air brake stopping distance formula

Estimated Time:1m 0s

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