In traffic engineering, what is Peak Hour Factor (PHF) used for?

• A. PHF equals the ratio of peak hour volume to hourly average flow.
• B. PHF is the ratio of average flow to peak hour volume.
• C. PHF is used to evaluate vehicle speeds in the corridor.
• D. PHF = (hourly average flow)^2 / (peak hour flow × peak hour volume).

Answer: (D)

Peak Hour Factor is about how traffic is spread within the busiest hour. It measures the uniformity of flow during that hour: if vehicles arrive at a steady rate, the factor is high (close to 1); if most of the volume comes in a short burst, the factor drops.

The form given, PHF = (hourly average flow)^2 / (peak hour flow × peak hour volume), encodes that idea by tying the overall hour’s average flow to how large the peak flow and the peak hour volume are. When traffic is evenly distributed, the peak values scale with the average, pushing PHF toward 1. When a sharp peak dominates the hour, the peak quantities grow relative to the square of the average, making PHF smaller. This factor helps engineers gauge whether results based on assuming uniform flow are realistic and informs decisions on capacity, pavement design, and signal timing.

Other options either describe different concepts (like speeds) or present a different, not-standard relationship for PHF, so they don’t capture how PHF reflects the within-hour variability of traffic.