Traffic engineering is full of nuanced concepts that help us better understand how roadways function under pressure—and one of the most important of these is the Peak Hour Factor (PHF). Whether you’re conducting a Traffic Impact Study (TIA), designing an intersection, or calibrating a simulation model, PHF plays a critical role in capturing the intensity and variability of traffic during peak conditions.
In this article, we’ll break down:
- What PHF is
- Why it matters in traffic studies
- How to calculate it. You can calculate the Peak Hour Factor using our free online PHF Calculator to quickly analyze traffic flow consistency.
- When and how to use it
- Tips and common pitfalls to avoid
🚦 What is Peak Hour Factor (PHF)?
PHF measures how evenly traffic volume is distributed across the peak hour. It helps identify whether traffic arrives consistently or in short, sharp bursts (i.e., peaks within the peak hour).
In more technical terms, PHF is the ratio of the total hourly volume to the peak 15-minute volume multiplied by 4. It reflects the “peaking” characteristics of traffic flow and is used to adjust capacity and level of service analyses.
🔢 PHF Formula
Where:
- Total Hourly Volume = number of vehicles in the highest 60-minute period
- Peak 15-minute Volume = number of vehicles in the highest 15-minute period within that same hour
💡 Example PHF Calculation
Suppose the traffic counts for an approach during the AM peak are as follows:
| Time Interval | Vehicles |
|---|---|
| 7:30 – 7:45 | 200 |
| 7:45 – 8:00 | 240 |
| 8:00 – 8:15 | 260 ← Peak 15 min |
| 8:15 – 8:30 | 220 |
- Total Hourly Volume = 200 + 240 + 260 + 220 = 920
- Peak 15-Min Volume = 260
So, the PHF is 0.88, indicating some degree of traffic concentration during the peak 15 minutes.
🎯 Why PHF Matters in Traffic Impact Studies
In Level of Service (LOS) and capacity analysis, we need to convert hourly traffic volumes into accurate short-term demand estimates. If traffic flows steadily, an assumption of even hourly distribution might work. But in reality, traffic often surges during a few high-demand minutes.
A low PHF (e.g., 0.80–0.85) signals higher peaking—meaning the infrastructure might need to handle more intense short-term volumes than the hourly rate suggests. Conversely, a PHF closer to 1.0 reflects even flow, which typically requires less conservative design assumptions.
📘 When and How to Use PHF
- Capacity Analysis: Use PHF when performing signal timing or intersection capacity assessments in Synchro or HCS.
- Simulation Models: While microsimulation tools like VISSIM use more granular demand inputs, you may still need PHF when summarizing volumes or calibrating models.
- Traffic Impact Studies: Include PHF when presenting hourly turning movement counts to show the level of peaking, especially when assessing mitigation measures or queuing concerns.
- Signal Design: A lower PHF may prompt increased green time allocation for a critical approach.
Typical PHF ranges (per FHWA and ITE guidelines):
- Arterials: 0.85–0.95
- Freeways: 0.88–0.98
- Urban intersections: 0.80–0.95
🧠 Pro Tips
- Always collect 15-minute interval data (or finer if possible). If you only have hourly volumes, PHF can’t be calculated.
- Don’t default to 1.0 — Unless supported by data, assuming a PHF of 1.0 can result in under-design.
- Compare across time periods — Use PHF for both AM and PM peaks to understand temporal traffic dynamics.
- Adjust for context — A site near a school or a place of worship might show sharper peaks than a business park or shopping plaza.
⚠️ Common Pitfalls
- Using daily volume instead of hourly volume in the PHF formula
- Relying on assumed values instead of actual observed data
- Ignoring PHF in intersection analysis, leading to misleading LOS or delay calculations
- Misinterpreting a low PHF — it doesn’t always mean congestion; it may reflect synchronized arrival patterns
📄 Final Thoughts
The Peak Hour Factor is more than just a formula—it’s a lens for understanding how stress is applied to road infrastructure over time. In Traffic Impact Studies, incorporating PHF ensures more accurate, realistic modeling of congestion, queues, and delays.
Whether you’re assessing an intersection, designing a development access, or preparing a site plan submission, always calculate and apply PHF where applicable—and explain it in your report to demonstrate a nuanced understanding of traffic flow behavior.
Need a pre-formatted template that integrates PHF calculations, signal warrant analysis, and capacity results?
🚧 Download our free Traffic Impact Study (TIA) Report Template — preloaded with placeholders, figures, and formatting to make your TIA workflow more efficient.
