A commercial motor vehicle equipped with an air brake system travels along a dry, level hi… — Commercial Driver's License (CDL) General Knowledge Exam

A commercial motor vehicle equipped with an air brake system travels along a dry, level highway at a constant speed of $60\text{ mph}$ ( $88\text{ ft/s}$ ). The driver spots a sudden road hazard with an average perception time of $1.75\text{ seconds}$ and takes $0.75\text{ seconds}$ of reaction time to move their foot to the brake pedal. The pneumatic air brake system has a brake lag delay of $0.50\text{ seconds}$ before air pressure activates the brake shoes, followed by an effective mechanical braking distance of $216\text{ feet}$ to bring the truck to a complete stop. What is the total stopping distance of the vehicle in feet?

Answer: 480 feet

Answer

480 feet

Total stopping distance for an air-brake commercial motor vehicle is calculated using the formula:

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

. At

60\text{ mph}

(

88\text{ ft/s}

), the truck covers

154\text{ ft}

during perception (

1.75\text{ s}

66\text{ ft}

during driver reaction (

0.75\text{ s}

), and

44\text{ ft}

during pneumatic brake lag (

0.50\text{ s}

). Adding these pre-braking distances (

264\text{ ft}

) to the mechanical braking distance (

216\text{ ft}

) yields exactly

480\text{ feet}

Step-by-Step Solution

Calculate perception distance

154 feet

88\text{ ft/s}

, the truck travels for

1.75\text{ s}

before the driver consciously sees and identifies the hazard (

88 \times 1.75 = 154\text{ ft}

Calculate reaction distance

66 feet

The vehicle continues at full speed for

0.75\text{ s}

while the driver physically moves their foot onto the brake pedal (

88 \times 0.75 = 66\text{ ft}

Calculate air brake lag distance

44 feet

Air brakes require roughly

0.50\text{ s}

for compressed air to flow through the lines to the brake chambers before mechanical friction begins (

88 \times 0.50 = 44\text{ ft}

Add the mechanical braking distance to the pre-braking distances

480 feet

Total stopping distance is the sum of Perception Distance (

154\text{ ft}

) + Reaction Distance (

66\text{ ft}

) + Brake Lag Distance (

44\text{ ft}

) + Braking Distance (

216\text{ ft}

), giving

154 + 66 + 44 + 216 = 480\text{ feet}

Key Concept

Total stopping distance for commercial vehicles equipped with air brakes includes four distinct sequential components: perception distance, reaction distance, air brake lag distance, and active mechanical braking distance.

Estimated Time:1m 15s

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