Design of Traffic Volume
Design of Traffic VolumeObjectivesTo determine the level of
serviceTo determine the number of lanes in designing a new roadway
to achieve the desired level of serviceThings to
ConsiderVariability in traffic volume a. time of dayb. day of
weekc. time of yeard. type of roadwayHourly volume for design
and/or analysis
From Figure 6.8
K1 = 0.148K30 = 0.12K = Hourly Volume as a proportion of
AADT
Highest K = K1 = 0.148
Highest K designates the worst traffic hour in the year. So it
would be wasteful to use resources in designing a roadway with
additional lanes to provide for a very rare occurrence.
US practice to use a design hour volume that is between the 10th
and 50th highest hourly volume.
In general, the 30th highest hourly volume is used to design
roadways.
Table 6.1 will be used for the computations in designing a
roadwayDDHV - directional design hour volumeD peak hour traffic
volume factorAADT annual average daily traffic
V volume of maximum service flowPHF peak hour factorN number of
lanes in one directionFhv heavy vehicle adjustment factorFp driver
population adjustment factorExampleA freeway is to be designed as a
passenger-caronly facility for an AADT of 35,000 vehicles per day.
It is estimated that the freeway will have a free-flow speed of 70
mi/h. The design will be for commuters, and the peak-hour factor is
estimated to be 0.85 with 65% of the peak-hour traffic traveling in
the peak direction. Assuming that Fig. 6.8 applies, determine the
number of lanes required to provide at least LOS C using the
highest annual hourly volume and the 30th highest annual hourly
volume.
GivenAADT = 35,000 veh/hD = 0.65K1 = 0.148K30 = 0.12PHF =
0.85FFS = 70 mi/hFhv = 1.0Fp = 1.0
RequiredNumber of lanes
Solution:1. determine DDHV2. determine maximum service flow rate
for LOS C in table 6.1 from given FFS3. determine N
SolutionDetermine DDHV
*Highest annual hourly volume *30th highest annual hourly
volume
SolutionDetermine maximum service flow for LOS C with given
FFS
FFS = 70 mi/h
Maximum flow rate = 1735 pc/h/lnSolutionDetermine N
ConclusionFor the Highest annual hourly volume, the roadway
needs to have 3 lanes in both directions, 6 lanes in total, to
provide a Level of Service C.
For the 30th highest annual hourly volume, the roadway needs to
have 2 lanes in both directions, 4 lanes in total, to provide a
Level of Service C.
