During a driver safety evaluation, an instructor evaluates the latency phases of stopping … — Commercial Driver's License (CDL) General Knowledge Exam

During a driver safety evaluation, an instructor evaluates the latency phases of stopping an air-brake-equipped commercial vehicle traveling at a constant speed of $45\text{ mph}$ ( $66\text{ ft/s}$ ) on a dry, level roadway. Assuming standard CDL baseline metrics (perception time of $1.75\text{ seconds}$ , driver reaction time of $0.75\text{ seconds}$ , and air brake lag of $0.5\text{ seconds}$ ), how far will the commercial vehicle travel from the instant a hazard appears until the brake linings physically contact the drums or rotors to begin deceleration, and which specific components constitute this distance?

$198\text{ feet}$ ; comprised of perception distance ( $115.5\text{ ft}$ ), reaction distance ( $49.5\text{ ft}$ ), and brake lag distance ( $33\text{ ft}$ ).Answer
B
$165\text{ feet}$ ; comprised of perception distance ( $115.5\text{ ft}$ ) and reaction distance ( $49.5\text{ ft}$ ), excluding mechanical delay.
C
$132\text{ feet}$ ; comprised of reaction distance ( $49.5\text{ ft}$ ) and brake lag distance ( $33\text{ ft}$ ), omitting perception time.
D
$264\text{ feet}$ ; calculated by applying the following-distance rule of $4\text{ seconds}$ total headway distance prior to brake application.

Answer

198\text{ feet}

; comprised of perception distance (

115.5\text{ ft}

), reaction distance (

49.5\text{ ft}

), and brake lag distance (

33\text{ ft}

Total stopping distance in an air-brake-equipped commercial vehicle is composed of four distinct components: perception distance, reaction distance, brake lag distance, and effective braking distance. The distance covered before the brakes physically engage consists of the three latency phases: perception (

1.75\text{ s} \times 66\text{ ft/s} = 115.5\text{ ft}

), reaction (

0.75\text{ s} \times 66\text{ ft/s} = 49.5\text{ ft}

), and air brake lag (

0.5\text{ s} \times 66\text{ ft/s} = 33\text{ ft}

). Summing these gives exactly

198\text{ feet}

of travel before the vehicle begins decelerating.

Step-by-Step Solution

Calculate perception distance

Perception distance

= 66\text{ ft/s} \times 1.75\text{ s} = 115.5\text{ ft}

Perception distance is the distance traveled from the moment the driver's eyes see a hazard until the brain recognizes it.

Calculate driver reaction distance

Reaction distance

= 66\text{ ft/s} \times 0.75\text{ s} = 49.5\text{ ft}

Reaction distance is the distance traveled while moving the foot from the accelerator pedal to pressing the brake pedal.

Calculate air brake lag distance

Brake lag distance

= 66\text{ ft/s} \times 0.5\text{ s} = 33\text{ ft}

Air brake systems require approximately

0.5\text{ seconds}

for air to flow through lines and build pressure in chambers to push linings against drums/rotors.

Sum all pre-braking distance components

Total pre-braking distance

= 115.5\text{ ft} + 49.5\text{ ft} + 33\text{ ft} = 198\text{ ft}

Mechanical deceleration only begins after perception, reaction, and brake lag phases are fully completed.

Key Concept

Air Brake Stopping Distance Components and Latency Phases

Estimated Time:2m 0s

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