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Difficulty: HardStopping Distance Components

While traveling at a sustained speed of 60 mph60\text{ mph} (88 ft/s88\text{ ft/s}) on a dry, level interstate, the driver of an air-brake-equipped commercial motor vehicle identifies a sudden lane blockage. The vehicle's foundation brakes require 210 ft210\text{ ft} of actual braking distance once the air pressure activates the brake shoes. Under standard commercial driving benchmarks—perception time of 1.75 s1.75\text{ s}, driver reaction time of 0.75 s0.75\text{ s}, and air brake lag of 0.5 s0.5\text{ s}—what is the total stopping distance required to bring the vehicle to a complete stop?

  1. A
    320 ft320\text{ ft}
  2. B
    430 ft430\text{ ft}
  3. 474 ft474\text{ ft}Answer
  4. D
    342 ft342\text{ ft}

Answer

474 ft474\text{ ft}
Total stopping distance for an air brake vehicle is calculated by summing all four distinct phases: Perception Distance (1.75 s×88 ft/s=154 ft1.75\text{ s} \times 88\text{ ft/s} = 154\text{ ft}), Reaction Distance (0.75 s×88 ft/s=66 ft0.75\text{ s} \times 88\text{ ft/s} = 66\text{ ft}), Brake Lag Distance (0.5 s×88 ft/s=44 ft0.5\text{ s} \times 88\text{ ft/s} = 44\text{ ft}), and Braking Distance (210 ft210\text{ ft}). Adding these four values yields 154+66+44+210=474 ft154 + 66 + 44 + 210 = 474\text{ ft}.

Step-by-Step Solution

1
Calculate the perception distance at 60 mph60\text{ mph} (88 ft/s88\text{ ft/s}).
1.75 s×88 ft/s=154 ft1.75\text{ s} \times 88\text{ ft/s} = 154\text{ ft}
Perception distance is the distance a commercial vehicle travels from the moment the driver's eyes see a hazard until the brain recognizes it.
2
Calculate the driver reaction distance.
0.75 s×88 ft/s=66 ft0.75\text{ s} \times 88\text{ ft/s} = 66\text{ ft}
Reaction distance is the distance traveled while the driver moves their foot from the accelerator pedal to the brake pedal.
3
Calculate the air brake lag distance.
0.50 s×88 ft/s=44 ft0.50\text{ s} \times 88\text{ ft/s} = 44\text{ ft}
Air brake systems require an additional half-second for air to flow through the lines and mechanical actuators to press the linings against the drums.
4
Sum all four individual distance components to determine total stopping distance.
154 ft+66 ft+44 ft+210 ft=474 ft154\text{ ft} + 66\text{ ft} + 44\text{ ft} + 210\text{ ft} = 474\text{ ft}
Total stopping distance for an air-brake-equipped vehicle is the sum of perception distance, reaction distance, brake lag distance, and actual braking distance.

Key Concept

Four-Component Air Brake Stopping Distance Formula
Estimated Time:2m 0s
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Topics

Stopping Distance Components

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