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 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)
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.