Calculator Inputs
Example Data Table
| Scenario | Facility | Lanes / Dir | Peak Volume (veh/h) | D-Split | PHF | HV % | Estimated FFS | Peak Capacity | LOS |
|---|---|---|---|---|---|---|---|---|---|
| Urban commuter peak | Freeway | 3 | 5400 | 58% | 0.92 | 8% | 63.5 mph | 5523 veh/h | C |
| Truck-heavy corridor | Freeway | 2 | 4200 | 60% | 0.88 | 18% | 58.2 mph | 3307 veh/h | F |
| Suburban arterial bypass | Multilane | 2 | 3200 | 55% | 0.93 | 6% | 49.8 mph | 3558 veh/h | B |
| Access-rich corridor | Multilane | 2 | 3800 | 57% | 0.90 | 10% | 43.6 mph | 3387 veh/h | D |
Formula Used
1) Adjusted free-flow speed
The calculator first estimates free-flow speed by subtracting geometric and access-related penalties from a base free-flow speed. The exact adjustments depend on whether the segment is analyzed as a freeway or a multilane highway.
2) Heavy-vehicle factor
Truck percentage and terrain influence passenger-car equivalency. A higher heavy-vehicle share lowers the effective flow quality and the estimated directional capacity.
3) Peak directional capacity
Capacity per lane is estimated from free-flow speed, then scaled by the number of lanes, peak-hour factor, heavy-vehicle factor, and driver population factor.
4) Passenger-car flow rate per lane
This converts the directional volume into a lane-based passenger-car equivalent demand rate used for density and LOS screening.
5) Density and LOS
Density is estimated by dividing lane demand by an estimated operating speed. The speed output is intentionally a screening estimate for quick engineering checks.
6) Capacity utilization
This shows how close the segment is to the estimated directional limit during the design peak hour.
How to Use This Calculator
- Choose the facility type that best matches the segment you are screening.
- Enter the geometry, directional demand, lane count, PHF, and heavy-vehicle share.
- For freeway analysis, pick the context that best represents the roadway environment.
- For multilane analysis, enter the posted speed limit, access-point density, and median type.
- Use the optional growth factor to test a future-year or near-term planning case.
- Press Analyze Highway Capacity to show results above the form.
- Review LOS, density, reserve capacity, and interpretation together rather than relying on a single output.
- Use the CSV and PDF buttons to save the result summary for reports or review meetings.
8 FAQs
1) What does this calculator estimate?
It screens freeway and multilane segment performance using geometry, peak demand, heavy vehicles, and adjustment factors. It is useful for planning, concept review, and quick operational checks.
2) Is this the same as a full project study?
No. It is a screening tool. Final design, interchange analysis, weaving evaluation, incident response planning, and environmental decisions still need corridor-level or project-level analysis.
3) Why does the calculator ask for directional split?
Peak traffic rarely divides equally between directions. The critical direction usually governs performance, so directional split converts total hourly demand into the controlling directional volume.
4) Why do heavy vehicles matter so much?
Trucks and buses accelerate differently, occupy more space, and reduce traffic stream quality. Their effect becomes stronger on rolling or mountainous terrain.
5) What is PHF?
PHF is the peak-hour factor. It reflects how sharply demand concentrates inside the hour. Lower PHF values indicate a more peaked demand pattern.
6) Can I use this for future-year scenarios?
Yes. Enter a growth factor to test a planning scenario. The tool increases the entered peak volume before computing directional demand and capacity usage.
7) What if the LOS is F?
LOS F means the screening density or demand level exceeds the estimated stable operating threshold. Consider demand management, operations strategies, or geometric improvements.
8) Why is the speed labeled as estimated?
Because this file provides a fast screening approximation. True operating speed under congestion depends on segment type, bottlenecks, control, incidents, and local calibration.