Traffic composition
 The effect of large or heavy vehicles in the traffic stream reduces
the maximum flow on the highway because of their operating characteristics; a heavy
vehicle occupies more space in the traffic stream than does a passenger car.
Grade
 The effect of a grade depends on the length and slope of the grade. Traffic
operations are significantly affected when grades of3% or greater are 0.25 mile or
greater in length and when grades of 3% or less are greater than 0.5 mile in length. The
effect of grades on heavy vehicles is much greater than on passenger cars.
Driver population
 A driver population consisting primarily of weekday commuters
will have significantly different behavior than a driver population consisting of drivers
unfamiliar with the roadway. For example, recreational traffic capacities can be as
much as 20% lower than commuter traffic capacities.
Interchange spacing

As interchanges are more closely spaced, the lengths of basic
freeway segments unaffected by interchanges decreases as the weaving movements at
interchanges have an increasing impact on traffic flow resulting in a reduction of
speeds.
912
Given: Freeway; design volume of 5000 veh/h; PHF: 0.9; trucks: 10%; design LOS: C;
free flow speed: 70 mi/h; no lateral obstructions; rolling terrain; interchange spacing: 3
mi.
Determine: Number of 12 ft lanes required in each direction.
Step 1: Compute passenger car equivalent flow rate for peak 15minute period using
Equation 9.21.
Determine fp(driver population factor); assume mainly commuter traffic, fp= 1.0
Determine ET(PCE for trucks), using Table 9.25; ET= 2.5
Determine
ER(PCE for RVs), using Table 9.25; ER
= 2.0
Determine
fHV(heavy vehicle adjustment
factor), using Equation
9.4
1
1
J:
v
=
=
=
0.87
H
I+Pr(Er
1)+PR(ER
1)
1+0.10(2.51)+0.00(2.01)
Step 2: Assume six lanes (three in each direction)
V
vp
=
 (5000)/(0.90)(3)(0.87)(1.0) = 2128 pc/h/ln
(PHF)(N)(IHV )(Ip)
Step 3: Compute free flow speed using Equation 9.23
FFS
=
BFFS

fiw

fie IN

lID
Determine fLwusing Table 9.34, fLw= 0.0
Determine fLcusing Table 9.35, fLc= 0.0
Determine fNusing Table 9.36, fN
= 3.0
Determine fIDusing Table 9.37, fID= 0.0
FFS
=
70 0.0
0.0
3.0
0.0
=
67.0 mi/h
91
Traffic and Highway Engineering, Third Edition: Instructor's Manual
Garber and Hoel
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Step 4: Compute average passenger car speed and density to determine LOS.
For FFS = 67 mi/h and vp= 2128 pc/h/1n,using Figure 9.9, the average passenger car
speed, S = 60.0 mi/h
Density
= (2128
pc/h/ln) /60.0 mi/h
= 35.4
pc/mi/ln
Using Table 9.33, this density corresponds to LOS E, therefore, a three lane section is
inadequate. Repeat steps 2 and 3.
Step 2: Assume eight lanes (four in each direction)
vp
=
V
 (5000)/(0.90)(4)(0.87)(1.0) = 1597 pc/h/ln
(PHF)(N)(IHV )(Ip)
.1i1
Step 3: Compute free flow speed using Equation 9.23
FFS
=
BFFS

fLw

fLc IN

lID
Determine fLwusing Table 9.34, fLw
= 0.0
Determine fLcusing Table 9.35, fLc= 0.0
Determine fNusing Table 9.36, fN
= 1.5
Determine fIDusing Table 9.37, fID
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 Spring '10
 EssamRadwan
 Household income in the United States, Garber, average passenger car, passenger car speed

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