The peak hour factor (PHF) is the hourly volume during the maximum-volume hour of the day divided by the peak 15-minute flow rate within the peak hour; a measure of traffic demand fluctuations within the peak hour.
The peak hour factor is used in HCM capacity and level of service analysis to account for the variation in traffic volumes during the peak hour. Following is an example of how the peak hour factor is computed and how it might affect the final results of a capacity calculation.
The table below shows flow rates that were measured for four 15-minute time periods for each of the 12 intersection movements. Examination of this table shows that second time period, which begins at 4:15 pm, is the peak 15-minute period of the four that are shown here. The total flow for this time period is 4,220 veh/15 minutes, or 16,880 veh/hr. The average flow rate for the hour is 12,640 veh/hr; this is the sum of the total volumes observed during each of the four 15-minute periods shown below. The peak hour factor can then be computed as follows:
PHF = (average flow rate)/(4*Peak 15 minute flow rate)
You can see that the possible values of PHF can range between 0.25 and 1.00, inclusive. Higher numbers indicate a flatter peak. It is rare that PHF drops much below a value of about 0.70. In this case, the PHF of 0.75 is indicative of a very sharp peak for an urban environment, and is probably more characteristics of small towns and cities than larger urban areas.