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Difficulty: MediumStopping Distance Components

When a commercial motor vehicle doubles its travel speed from 30 mph30\text{ mph} to 60 mph60\text{ mph}, its perception and reaction distances double, while its actual braking distance increases by approximately four times.

Answer: Answer

Answer

True. Perception and reaction distances increase linearly with vehicle speed, whereas braking distance increases with the square of the speed.
The statement accurately identifies that pre-braking distance components (perception and reaction) scale linearly with speed, whereas mechanical braking distance scales quadratically with speed due to kinetic energy quadrupling when velocity doubles.

Step-by-Step Solution

1
Analyze the human-factor components of stopping distance (perception distance and reaction distance).
Perception distance and reaction distance are calculated as speed multiplied by constant reaction/perception time intervals (d=v×td = v \times t). Doubling speed from 30 mph30\text{ mph} to 60 mph60\text{ mph} results in exactly twice the distance covered during these time intervals.
Human perception and reaction times remain relatively constant for an alert driver, making the distance traveled during these phases directly proportional to speed.
2
Analyze the mechanical braking distance component.
Braking distance depends on absorbing the vehicle's kinetic energy (Ek=12mv2E_k = \frac{1}{2}mv^2). When speed doubles, kinetic energy increases by 22=42^2 = 4 times, requiring approximately four times the braking distance to stop.
Brakes stop a commercial vehicle by converting kinetic energy into thermal energy through friction, so distance scales with the square of velocity.
3
Evaluate the complete statement against the identified component behaviors.
The statement correctly distinguishes the linear increase of perception/reaction distance from the fourfold (quadratic) increase in braking distance.
Both parts of the statement accurately reflect CDL stopping distance principles.

Key Concept

Effect of speed scaling on individual stopping distance components
Estimated Time:1m 0s
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Topics

Stopping Distance Components

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