Practice question

What does Peak Hour Factor (PHF) indicate in traffic counts?

  1. How evenly traffic is distributed within the hour; higher PHF means more uniform flow.
  2. The maximum average speed during peak hour.
  3. The percentage of time the signal is green during the hour.
  4. The time headway between every two vehicles.

Answer: A — How evenly traffic is distributed within the hour; higher PHF means more uniform flow.

Peak Hour Factor is about how traffic is spread out within the hour. It’s defined as the hourly volume divided by four times the largest 15-minute volume observed in that hour. If the traffic is evenly distributed, the busiest 15-minute period would be about a quarter of the hourly total, giving a PHF close to 1. If most of the traffic occurs in a short subinterval, the largest 15-minute block is larger relative to the hour, and PHF drops below 1. So a higher PHF means more uniform flow throughout the hour, while a lower PHF indicates a more peaked, uneven distribution. This concept is not about speed, signal timing, or headways; it specifically captures the distribution of flow over the hour. For example, with an hourly volume of 1000 vehicles, a busiest 15-minute block of 250 yields PHF = 1, while a busiest 15-minute block of 400 yields PHF = 1000 / (4*400) = 0.625.

Practice more: ITE Traffic Bowl Fundamentals and Engineering Practice Test →