The Peak Hour Factor (PHF) measures the relationship between the peak 15-minute interval of traffic compared to the total vehicle volume over the entire Peak Hour. It represents how constant vehicle volumes are during the Peak Hour.
The PHF is a ratio of the total hourly traffic volume against the the busiest 15-minute interval. The peak 15-minute interval is represented for the entire hour.
PHF = (total hourly volume) / [(peak 15-minute volume within the hour x 4)]
= V / (VPeak x 4)
Where: 0 < PHF >= 1
A PHF that is equal to 1 demonstrates that the traffic volume in every 15-minute interval is the same and therefore the traffic flow is constant throughout the hour. As the PHF decreases, it shows that the traffic flows are more variable and the traffic volume has a spike during the peak 15-minute interval.
Example 1: The 15-minute vehicle volumes are relatively constant:
- V15 = 190
- V30 = 200
- V45 = 190
- V60 = 190
PHF = (190 + 200 + 190 + 190) / [(200 x 4)] = 0.96
Example 2: The 15-minute vehicle volumes are more variable:
- V15 = 175
- V30 = 200
- V45 = 150
- V60 = 125
PHF = (175 + 200 + 150 + 125) / [(200 x 4)] = 0.81