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Peak Hour Factor Calculator — PE Civil Transportation Worked Example
The peak hour factor converts an hourly volume into the rate the design must actually serve: the flow of the worst 15 minutes, expressed as an hourly rate. Enter the hourly volume and the peak 15-minute count.
For study and checking only. This calculator runs entirely in your browser — nothing is sent anywhere. It is meant to verify your hand calculations while you study; on the exam you will work by hand with the NCEES reference handbook.
Quick answer: the PHF definition
\[\text{PHF} = \frac{V}{4\,V_{15}} \qquad \text{peak flow rate} = \frac{V}{\text{PHF}} = 4\,V_{15}\]
PHF is always at most 1.0 — a value above 1 means the 15-minute count and the hourly volume have been swapped or the count is not from the peak hour. The flatter the demand inside the hour, the closer PHF sits to 1.
How to use it — and how to sanity-check the answer
Use the largest 15-minute count inside the same hour as V — the four quarter-hours of the peak hour, not the four largest of the day. Sanity-check with the identity: peak rate = 4 × V15 exactly, so the two routes to the peak rate must agree.
Worked example (reproduces in the tool above)
Worked example PHF and peak flow rate
Given: An approach carries an hourly volume \(V = 1{,}840\) veh/h, and its peak 15-minute count is \(V_{15} = 540\) veh.
Solution:
- \(\text{PHF} = \dfrac{1{,}840}{4 \times 540} = \dfrac{1{,}840}{2{,}160} = 0.852\).
- Peak flow rate \(= V/\text{PHF} = 1{,}840/0.852 = 2{,}160\) veh/h — the same as \(4 \times 540\), as it must be.
Answer: PHF \(= 0.852\); peak flow rate \(= 2{,}160\) veh/h.