Definition of peak hour factor in traffic engineering?

Peak hour is the period during which traffic volume is at its highest. The analysis of level of service is based on peak rates of flow occurring within the peak hour because substantial short-term fluctuations typically occur during an hour. Common practice is to use a peak 15-minute rate of flow. Flow rates are usually expressed in vehicles per hour, not vehicles per 15 minutes. The relationship between the peak 15-minute flow rate and the full hourly volume is given by the peak-hour factor (PHF) as shown in the following equation:

PHF = (Hourly volume) / (peak rate of flow within the hour)

If 15-minute periods are used, the PHF is computed as: PHF = V / (4 x V15)

Where

V = peak-hour volume (vph)

V15 = volume during the peak 15 minutes of flow (veh/15 minutes)

V15 is multiplied by 4 because 4 x 15 = 60 minutes. PHF tells you how close the peak hour volume is to the "maximum" volume (if all the periods within the hour had the same volume as the busiest one).