Regulatory speed limits for roads are commonly set with reference to which percentile of the observed spot speed distribution (cumulative frequency basis)?

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:

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