The following excerpts were taken from the 1994 Highway Capacity
Manual, published by the Transportation Research Board.
Peak Hour and Design Hour
Capacity and other traffic analyses focus on the peak hour of traffic volume, because
it represents the most critical period for operations and has the highest capacity
requirements. The peak hour volume, however, is not a constant value from day to day or
from season to season.
If the highest hourly volumes for a given location were listed in descending order, a
large variation in the data would be observed, depending on the type of route and facility
under study.
Rural and recreational routes often show a wide variation in peak-hour volumes. Several
extremely high volumes occur on a few selected weekends or other peak periods, and traffic
during the rest of the year is at much lower volumes, even during the peak hour. This
occurs because the traffic stream consists of few daily or frequent users; the major
component of traffic is generated by seasonal recreational activities and special events.
Urban routes, on the other hand, show little variation in peak-hour. . . .
The relationship between the 15-min flow rate and the full hourly volume is given by
the peak hour factor, defined in Part A of this chapter (see below).
Whether the design hour was measured, established from the analysis of peaking
patterns, or based on modeled demand, the peak-hour factor (PHF) is applied to determine
design hour flow rates.
Peak-hour factors in urban areas generally range between 0.80 and 0.98. Lower values
signify greater variability of flow within the subject hour, and higher values signify
little flow variation. Peak-hour factors over 0.95 are often indicative of high traffic
volumes, sometimes with capacity constraints on flow during the peak hour.
(Description of PHF from Part A, as referred to above.)
Peak rates of flow are related to hourly volumes through the use of the peak-hour
factor. This factor is defined as the ratio of total hourly volume to the peak rate of
flow within the hour:
PHF = Hourly volume/Peak rate of flow (within the hour)
If 15-min periods are used, the PHF may be computed as
PHF = V/(4 x V15)
Where
PHF = peak-hour factor,
V = hourly volume (vph), and
V15 = volume during the peak 15 min of the peak hour (veh/15 min).
Where the peak-hour factor is known, it may be used to convert a peak-hour volume to a
peak rate of flow, as follows:
v = V/PHF (2-3)
Where
v = rate of flow for a peak 15-min period (vph),
V = peak-hour volume (vph), and
PHF = peak-hour factor.
Equation 2-3 need not be used to estimate peak flow rates where traffic counts are
available. The chosen count interval must allow the identification of the maximum 15-min
flow period. The rate may then be directly computed as 4 times the maximum 15-min count.
Many of the procedures use this conversion to allow computations to focus on the peak
flow period within the peak hour.