What is the Peak Hour Factor (PHF)?

PHF is the ratio of total hourly volume to the peak flow rate extrapolated from the highest 15-minute count. It measures how uniformly traffic is distributed within the peak hour. Values range from 0.25 (all traffic in one 15-min period) to 1.0 (perfectly uniform flow).

Why use four 15-minute intervals?

The peak hour is divided into four equal 15-minute periods because 15 minutes is the standard counting interval used in traffic engineering and the Highway Capacity Manual (HCM). Shorter intervals can introduce too much statistical noise.

What is a typical PHF value?

Urban arterials typically see PHF values between 0.80 and 0.98. Rural highways tend to be lower (0.70–0.85) due to more sporadic traffic. Values below 0.70 suggest highly peaked demand and may warrant separate peak-period analysis.

How does PHF affect capacity analysis?

The peak flow rate used in HCM capacity analysis equals hourly volume divided by PHF. A lower PHF increases the design flow rate, which may require higher lane capacity or adjusted signal green time to avoid oversaturation.

What if my counts are zero or very low?

If the highest 15-minute count is zero, PHF is undefined (no traffic). Very low counts yield high variability; collect counts over multiple days and average them before computing PHF for design purposes.

Peak Hour Factor Calculator

Name: Peak Hour Factor Calculator
Availability: InStock
Author: Nham Vu

Enter four 15-minute traffic volume counts from the peak hour to calculate the Peak Hour Factor (PHF).

15-Minute Traffic Counts

Enter vehicle counts for each 15-minute interval within the peak hour.

Interval 1 — Q1 (e.g., :00–:15)

Interval 2 — Q2 (:15–:30)

Interval 3 — Q3 (:30–:45)

Interval 4 — Q4 (:45–:60)

Formula (HCM)

PHF = PHV ÷ (4 × V_peak15)

PHV = Q1 + Q2 + Q3 + Q4 | V_peak15 = max(Q1…Q4)

Results

Enter counts and click Calculate

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Summary

Enter four 15-minute traffic volume counts from the peak hour to calculate the Peak Hour Factor (PHF).

How it works

Observe traffic during the peak hour and record vehicle counts for each of the four 15-minute intervals.
Enter the four 15-minute counts (Q1 through Q4) in the input fields.
The calculator sums them to find the peak-hour volume (PHV).
It identifies the highest single 15-minute count and multiplies by 4 to get the peak flow rate.
PHF = PHV ÷ (4 × peak 15-min count). A result close to 1.0 means uniform flow; lower values indicate peaky demand.

Use cases

Adjust intersection capacity analysis using HCM methodology.
Set signal timing plans that account for uneven demand within the hour.
Evaluate whether a road segment needs capacity improvements.
Calibrate traffic simulation models with realistic peaking behavior.
Support traffic impact studies for development permit applications.
Compare PHF values across multiple study hours or locations.

Frequently Asked Questions

Last updated: 2026-07-24 · Reviewed by Nham Vu

Peak-Hour Volume (PHV)	—
Peak 15-min Count (V_peak15)	—
Peak 15-min Interval	—
Peak Flow Rate (veh/h)	—
Demand Uniformity	—