Daily Question: Rate of Flow Based on Traffic Density

NCEES Civil PE Specification VI.C.: 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 😃

INCORRECT 😟

Average:

46.6%

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:
$$ v=(18\ mph)(175\ vpm)=3150\ vph $$