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Difficulty: HardStopping Distance Components

When a commercial motor vehicle's travel speed doubles from 25 mph25\text{ mph} to 50 mph50\text{ mph}, each of the four individual components of total stopping distance—perception distance, reaction distance, air brake lag distance, and braking distance—increases by a factor of four (4×4\times).

Answer: Answer

Answer

False
The statement is false because doubling vehicle speed results in a two-fold (2×2\times) increase for perception distance, reaction distance, and air brake lag distance, while only the mechanical braking distance increases four-fold (4×4\times).

Step-by-Step Solution

1
Analyze how speed scaling affects the pre-braking distance components (perception, reaction, and air brake lag).
Perception distance (1.75 s1.75\text{ s}), driver reaction distance (0.75 s0.75\text{ s}), and air brake lag distance (0.5 s0.5\text{ s}) are determined by fixed time durations multiplied by vehicle speed (d=v⋅td = v \cdot t). When speed doubles from 25 mph25\text{ mph} to 50 mph50\text{ mph}, each of these three components increases by exactly two times (2×2\times).
Because driver perception time, reaction time, and pneumatic signal transmission times remain constant, the distance traveled during these intervals is directly proportional (linear) to speed.
2
Analyze how speed scaling affects the mechanical braking distance component.
Braking distance is governed by kinetic energy (Ek=12mv2E_k = \frac{1}{2}mv^2) that must be dissipated by brake friction work (W=F⋅dW = F \cdot d). Doubling speed results in (2)2=4(2)^2 = 4 times the kinetic energy, requiring four times (4×4\times) the mechanical braking distance.
Kinetic energy increases with the square of the speed, creating a quadratic relationship for the braking distance component.
3
Evaluate the accuracy of the overall statement.
Only one component (braking distance) increases fourfold, whereas the other three components merely double.
The statement incorrectly claims that all four individual components increase by a factor of four.

Key Concept

Linear vs. Quadratic Scaling of Stopping Distance Components
Estimated Time:1m 30s
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Topics

Stopping Distance Components

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