Traffic Engineering MCQ with answer and solution

Concept

Percentile speeds and their applications in highway engineering

Formula

NONE

Step-by-step solution

1. Examine percentile speeds and their specific engineering applications: - 98th percentile speed: Standard design speed adopted for designing new highway geometric elements. - 85th percentile speed: Operating speed used for checking existing roadway geometric elements and setting safe speed limits. - 50th percentile speed: Median speed representing typical traffic flow. - 15th percentile speed: Used to establish minimum regulatory speed limits to avoid congestion. 2. Therefore, to check existing roadway geometric design elements, the 85th percentile speed is used. Answer: C

Explanation

In traffic engineering studies, the 85th percentile speed is defined as the speed at or below which 85% of vehicles travel under free-flowing conditions. It is universally adopted to establish regulatory safe speed limits and to check whether existing highway geometric design elements (sight distance, curvature, superelevation) accommodate prevailing operating speeds. The 98th percentile speed is used for the geometric design of new facilities, the 50th percentile is the median speed, and the 15th percentile establishes minimum speed limits.

Why the other options are wrong

A) Incorrect: 15th percentile speed is used for lower speed limits to prevent slow-moving vehicles from impeding traffic. B) Incorrect: 50th percentile speed is median speed. D) Incorrect: 98th percentile speed is the design speed used for planning and designing new highway facilities.

Shortcut

New design = 98th percentile; Checking existing roadway / safe speed limit = 85th percentile.

Common mistake

Selecting 98th percentile speed, which is the standard for designing new highways rather than evaluating existing facilities.