Work out peak hour factor instantly with clear inputs, formula shown and shareable results.
The peak hour factor is hourly volume divided by four times the busiest fifteen minute count. It measures how evenly demand is spread: a factor of 1.0 means perfectly uniform flow, while 0.8 means the busiest quarter hour carries a quarter more than its share. Capacity analysis uses the flow rate, four times the fifteen minute count, not the hourly volume.
Peak hour factor
PHF = hourly volume / (4 x highest 15 minute volume)
Design flow rate
Flow rate = hourly volume / PHF = 4 x peak 15 minute volume
PHF = hourly volume / (4 x highest 15 minute volume). The peak hour factor is hourly volume divided by four times the busiest fifteen minute count.
Because queues form during the peak quarter and do not clear. Designing on the hourly average leaves the intersection oversaturated for fifteen minutes every peak.
This calculator takes 2 inputs: Volume in the peak hour, Volume in the busiest fifteen minutes. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.