Speed zoning – lower bound on high-speed roads: The minimum permissible (posted) speed on high-speed roads is typically decided with reference to which percentile of the cumulative speed distribution?

Introduction / Context:
Speed zoning uses observed speed distributions to set rational limits. The upper limit is often related to the 85th percentile speed, balancing safety and operational efficiency. A lower bound (minimum posted speed) can be tied to the lower tail of the distribution to discourage excessively slow vehicles that disrupt flow and increase speed variance.

Given Data / Assumptions:

• High-speed road context (divided highways/access-controlled facilities).
• Speed data summarized as a cumulative distribution (percentile speeds).
• Objective: identify the commonly referenced percentile for minimum speed.

Concept / Approach:
In many design and traffic control references, the 85th percentile is used for upper limits, while the 15th percentile is referenced as a reasonable basis for lower bounds to reduce speed dispersion. This approach promotes homogeneity of speeds, enhancing safety.

Step-by-Step Solution:
Recognize percentile usage: 85th for upper limits, 15th for lower limits.Apply to high-speed corridors, where very slow vehicles are hazardous.Select the 15th percentile for the minimum permissible speed basis.

Verification / Alternative check:
Traffic engineering texts and many exam syllabi illustrate speed zoning with the 85th–15th percentile framework to bound operational speeds rationally.

Why Other Options Are Wrong:

• 20th, 30th, 40th percentiles: sometimes cited in local practices, but the most common teaching/example basis pairs 85th with 15th.
• None of these: incorrect because a typical reference percentile is known.

Common Pitfalls:

• Treating percentile guidance as rigid law; actual posting considers roadway, traffic mix, and safety data.
• Ignoring heavy-vehicle percentages that shift observed distributions.

Final Answer:
15th percentile