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.
Select an Answer:
CORRECT 😃
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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:
$$ v=(18\ mph)(175\ vpm)=3150\ vph $$
