Regulatory speed limits for roads are commonly set with reference to which percentile of the observed spot speed distribution (cumulative frequency basis)?
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A60th percentile speed
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B75th percentile speed
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C80th percentile speed
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D85th percentile speed
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E
Answer
Correct Answer: 85th percentile speed
Explanation
Introduction / Context:Speed zoning and the posting of statutory limits rely on the distribution of observed free-flow speeds. The chosen percentile should reflect a balance between safety, driver expectancy, and enforcement practicality.
Given Data / Assumptions:
- Use of spot speed studies conducted under free-flow, representative conditions.
- Objective is the typical rule-of-thumb used by many standards and agencies.
Concept / Approach:The 85th percentile speed is the speed at or below which 85% of drivers travel. It correlates well with the speed most drivers perceive as safe and reasonable for prevailing conditions, reducing speed variance and conflicts.
Step-by-Step Solution:
Collect spot speeds → sort and form cumulative frequency distribution.Read the 85th percentile speed from the curve.Use it as a basis for setting or checking posted limits, with engineering judgment and constraints (schools, work zones).Verification / Alternative check:Cross-check with crash history, roadside environment, and design speed to confirm selected limit.
Why Other Options Are Wrong:60th, 75th, and 80th percentiles are less representative of the prevailing safe speed and can increase non-compliance.
Common Pitfalls:Using mean speed instead of percentile; collecting data under congested or non-representative conditions.
Final Answer:85th percentile speed