Geometric Design MCQ with answer and solution

Concept

Design speed percentile for highway design

Formula

NONE

Step-by-step solution

1. Spot speed measurements are analyzed statistically to determine percentile speeds for traffic design and regulation. 2. While the 85th percentile speed is used for setting speed limits, the 98th percentile speed is used for highway geometric design. 3. Designing for the 98th percentile speed ensures safety and efficiency for almost all highway drivers. Answer: D

Explanation

The 98th percentile speed is defined as the speed at or below which 98 percent of vehicles travel under free-flowing conditions. Using this speed ensures that highway geometric features are safe for virtually all drivers while preventing unnecessarily costly over-design for extreme speeders. Consequently, the 98th percentile speed serves as the standard design speed for highway geometry.

Why the other options are wrong

A) The 15th percentile speed is used as the lower speed limit for traffic flow regulation. B) The 50th percentile speed represents the median traffic speed. C) The 85th percentile speed is commonly used to establish safe speed limits rather than geometric design speed.

Shortcut

98th percentile speed = Geometric design; 85th percentile = Speed limit.

Common mistake

Confusing the highway design speed percentile (98th percentile) with the safe speed limit percentile (85th percentile).