A fleet safety manager is evaluating the stopping capabilities of two fully loaded commerc… — Commercial Driver's License (CDL) General Knowledge Exam

A fleet safety manager is evaluating the stopping capabilities of two fully loaded commercial vehicles traveling at exactly $60\text{ feet per second}$ (approximately $41\text{ mph}$ ) on a level highway. Vehicle X is a heavy straight truck equipped with hydraulic brakes, while Vehicle Y is a heavy-duty tractor-trailer equipped with air brakes. Both drivers exhibit the standard average perception time of $1.75\text{ seconds}$ and the standard average reaction time of $0.75\text{ seconds}$ . Due to their heavy loads, the mechanical braking distance for both vehicles is exactly $140\text{ feet}$ . Based on standard commercial driving calculations, which of the following correctly identifies Vehicle Y's perception distance, its total stopping distance, and why its total stopping distance differs from Vehicle X?

Vehicle Y has a perception distance of $105\text{ feet}$ and a total stopping distance of $320\text{ feet}$ ; it requires $30\text{ feet}$ more to stop than Vehicle X due to air brake lag distance.Answer
B
Vehicle Y has a perception distance of $45\text{ feet}$ and a total stopping distance of $320\text{ feet}$ ; it requires $30\text{ feet}$ more to stop than Vehicle X due to air brake lag distance.
C
Vehicle Y has a perception distance of $105\text{ feet}$ and a total stopping distance of $290\text{ feet}$ ; its total stopping distance is identical to Vehicle X because both have the same mechanical braking distance.
D
Vehicle Y has a perception distance of $105\text{ feet}$ and a total stopping distance of $215\text{ feet}$ ; it requires $30\text{ feet}$ more to stop than Vehicle X due to air brake lag distance.

Answer

Vehicle Y has a perception distance of

105\text{ feet}

and a total stopping distance of

320\text{ feet}

; it requires

30\text{ feet}

more to stop than Vehicle X due to air brake lag distance.

The correct answer accurately calculates perception distance using the standard

1.75\text{-second}

time (

60 \times 1.75 = 105\text{ feet}

). It then correctly sums all four components for the air-braked Vehicle Y: perception (

105\text{ ft}

), reaction (

45\text{ ft}

), brake lag (

30\text{ ft}

), and braking (

140\text{ ft}

) for a total of

320\text{ feet}

. Finally, it correctly identifies that Vehicle Y takes

30\text{ feet}

longer to stop than the hydraulically-braked Vehicle X exclusively because hydraulic systems lack the air brake lag component.

Step-by-Step Solution

Calculate the perception distance for Vehicle Y.

60\text{ ft/s} \times 1.75\text{ s} = 105\text{ feet}

Perception distance is the distance traveled during the

1.75\text{-second}

average time it takes for the brain to recognize a hazard.

Calculate the reaction distance for Vehicle Y.

60\text{ ft/s} \times 0.75\text{ s} = 45\text{ feet}

Reaction distance is the distance traveled during the

0.75\text{-second}

average time it takes to physically move the foot to the brake pedal.

Calculate the air brake lag distance for Vehicle Y.

60\text{ ft/s} \times 0.50\text{ s} = 30\text{ feet}

Air brakes have an inherent mechanical lag of approximately

0.50\text{ seconds}

before the air flows and the brake shoes apply pressure.

Calculate the total stopping distance for Vehicle Y (air brakes).

105\text{ ft} + 45\text{ ft} + 30\text{ ft} + 140\text{ ft} = 320\text{ feet}

The total stopping distance for an air-braked vehicle consists of perception, reaction, brake lag, and mechanical braking distances.

Calculate the total stopping distance for Vehicle X (hydraulic brakes) and determine the difference.

Vehicle X stopping distance =

105\text{ ft} + 45\text{ ft} + 140\text{ ft} = 290\text{ feet}

. The difference is exactly the

30\text{-foot}

brake lag.

Hydraulic braking systems engage almost instantaneously and do not have the brake lag component found in air brake systems.

Key Concept

Air Brake vs. Hydraulic Brake Stopping Distance Components

Question

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Key Concept

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