Practice Question: Headway and Hourly Volume on a Two-Lane Road

NCEES Civil PE Specification VI.C.: Traffic Volume

Problem

The pictured two-lane roadway is congested in the direction of Lane A every evening during rush hour. A study was conducted, and Lane A was found to have a peak hour factor of 0.75 and a peak rate of flow of 150 vehicles per 15 minutes. Given that Lane B has an hourly volume during rush hour equal to one-third the volume of Lane A, determine the average vehicle headway anticipated for Lane B during rush hour in seconds per vehicle.

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Solution

Answer: B) 24 seconds per vehicle

Peak hour factor for Lane A:

$$ PHF_A=0.75 $$
Peak hourly flow for Lane A:
$$ v_{peak_A}=150\frac{veh}{15\ min}\ast4\frac{15\ min}{1\ hr}=600\ veh/hr $$
Hourly volume for Lane A:
$$ PHF_A=\frac{V_{hourly_A}}{v_{peak_A}} $$
$$ 0.75=\frac{V_{hourly_A}}{600\frac{veh}{hr}} $$
$$ V_{hourly_A}=450\frac{veh}{hr} $$
Hourly volume for Lane B:
$$ V_{hourly_B}=\frac{V_{hourly_A}}{3}=150\frac{veh}{hr} $$
Hourly volume to headway relationship:
$$ V_{hourly_B}=\frac{3600\frac{sec}{hr}}{headway_b} $$
Average headway at Lane B:
$$ headway_b=24\frac{sec}{vehicle} $$