A combination vehicle equipped with an air brake system travels along an expressway at a c… — Commercial Driver's License (CDL) General Knowledge Exam

A combination vehicle equipped with an air brake system travels along an expressway at a constant speed of $45\text{ mph}$ ( $66\text{ ft/s}$ ). The driver requires $1.75\text{ seconds}$ for hazard perception, $0.75\text{ seconds}$ for driver reaction, and $0.5\text{ seconds}$ for air brake lag before the brake linings contact the drums and begin slowing the vehicle. What is the total distance, in feet, traveled by the vehicle from the moment the hazard appears until the mechanical brakes actively engage?

Answer: 198 ft

Answer

198 feet

Total stopping distance in an air-brake-equipped commercial vehicle comprises perception distance, reaction distance, brake lag distance, and effective braking distance. Prior to actual vehicle deceleration, the vehicle travels during 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 three pre-braking components yields a total distance of

198\text{ feet}

Step-by-Step Solution

Determine the total pre-braking time by adding perception, reaction, and air brake lag durations.

1.75\text{ s} + 0.75\text{ s} + 0.50\text{ s} = 3.00\text{ seconds}

Before the vehicle begins mechanical deceleration, distance is accumulated across three distinct pre-braking phases.

Calculate the individual component distances traveled at

66\text{ ft/s}

Perception distance =

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

; Reaction distance =

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

; Brake lag distance =

0.50\text{ s} \times 66\text{ ft/s} = 33.0\text{ ft}

Distance is the product of velocity and elapsed time for each phase.

Sum the component distances to find the total distance traveled before braking starts.

115.5\text{ ft} + 49.5\text{ ft} + 33.0\text{ ft} = 198\text{ feet}

The total distance before active deceleration is the sum of perception distance, reaction distance, and brake lag distance.

Key Concept

Pre-braking stopping distance components for air brake vehicles include perception distance, reaction distance, and pneumatic brake lag distance.

Estimated Time:1m 0s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic