During a post-collision event data analysis on a dry, level highway, telematics indicate t… — Commercial Driver's License (CDL) General Knowledge Exam

During a post-collision event data analysis on a dry, level highway, telematics indicate that a fully loaded commercial motor vehicle equipped with air brakes was traveling at a steady speed of $60\text{ mph}$ ( $88\text{ ft/s}$ ). The driver required $1.75\text{ seconds}$ to perceive a roadway hazard, $0.75\text{ seconds}$ to physically apply the foot brake, experienced the typical $0.5\text{ second}$ air brake lag before the brakes engaged, and the vehicle required $250\text{ feet}$ of actual braking distance to come to a halt. What is the total stopping distance required for this vehicle from the initial perception of the hazard to a complete stop?

A
$470\text{ feet}$
$514\text{ feet}$ Answer
C
$360\text{ feet}$
D
$294\text{ feet}$

Answer

514\text{ feet}

Total stopping distance for an air-brake equipped commercial motor vehicle is composed of four distinct sequential components: Perception Distance + Reaction Distance + Brake Lag Distance + Braking Distance. At a speed of

60\text{ mph}

(

88\text{ ft/s}

), the vehicle travels

154\text{ feet}

during perception (

1.75\text{ s}

66\text{ feet}

during reaction (

0.75\text{ s}

44\text{ feet}

during air brake lag (

0.5\text{ s}

), and

250\text{ feet}

during active braking. Adding these four values together (

154\text{ ft} + 66\text{ ft} + 44\text{ ft} + 250\text{ ft}

) results in a total stopping distance of

514\text{ feet}

Step-by-Step Solution

Calculate the perception distance traveled during the

1.75\text{ second}

hazard recognition interval.

1.75\text{ s} \times 88\text{ ft/s} = 154\text{ feet}

Perception distance is the distance traveled from the instant the hazard becomes visible until the driver's brain recognizes it.

Calculate the reaction distance traveled during the

0.75\text{ second}

pedal movement interval.

0.75\text{ s} \times 88\text{ ft/s} = 66\text{ feet}

Reaction distance is the distance traveled while the driver moves their foot from the accelerator to the brake pedal.

Calculate the brake lag distance traveled during the

0.5\text{ second}

pneumatic activation delay.

0.5\text{ s} \times 88\text{ ft/s} = 44\text{ feet}

Commercial vehicles equipped with air brakes experience a mechanical delay of approximately half a second for air pressure to reach the brake chambers and engage the drums.

Sum all four components (perception, reaction, brake lag, and braking distance) to determine the total stopping distance.

154\text{ ft} + 66\text{ ft} + 44\text{ ft} + 250\text{ ft} = 514\text{ feet}

Total stopping distance for air brake vehicles is the comprehensive sum of all human and mechanical phases required to bring the vehicle to a full stop.

Key Concept

Four Components of Total Stopping Distance for Air Brake Commercial Vehicles

Estimated Time:1m 30s

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