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)

      =12,640/16,880

      =0.75

Time period

 Eastbound Westbound Northbound

Southbound

 Total
LT TH RT LT TH RT LT TH RT LT TH RT
4:00 pm 40 55 175 50 50 75 120 815 45 40 700 55 2,220
4:15 pm 50 75 375 55 80 125 215 1,025 20 60 1,975 165 4,220
4:30 pm 30 75 125 45 75 115 20 975 35 55 1,200 145 2,895
4:45 pm 45 60 175 55 85 150 145 1,015 45 50 1,350 130 3,305

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.