Practice Question: Rate of Flow Based on Traffic Density

NCEES Civil PE Specification Geometrics: Traffic volume

Problem

The northbound lane of the two-lane road shown has a free-flow speed of 60 mph and a jam density of 250 vehicles per mile. If the average density during rush hour is 175 vehicles per mile, determine the rate of flow in vehicles per hour.

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% Students Correctly Answered:

47.0%

Solution

Answer: B) 3150 vph

The rate of flow in vehicles per hour can be found by multiplying speed by density:
$$ v=SD $$
Speed can be determined based on free flow speed, density, and jam density.
$$ S=FFS\left(1-\frac{D}{D_j}\right) $$
Free flow speed (miles per hour):
$$ FFS\ =60\ mph $$
Density (vehicles per mile):
$$ D=175\ vpm $$
Jam Density (vehicles per mile):
$$ D_j=250\ vpm $$
Calculate speed:
$$ S=60\ mph\ast\left(1-\frac{175\ vpm}{250\ vpm}\right)=18\ mph $$
Calculate rate of flow:
$$ v=(18\ mph)(175\ vpm)=3150\ vph $$