Inputs
Example data table
| Case | Ventry (veh/h) | Qc (veh/h) | A | B | Entry lanes | Capacity (veh/h) | v/c | Delay (s/veh) | LOS |
|---|---|---|---|---|---|---|---|---|---|
| Example | 300 | 900 | 1130 | 0.001 | 1 | 383.9 | 0.849 | 26.4 | C |
Formula used
This calculator applies an exponential gap-acceptance style form for entry capacity:
- Clane = A · e-B·Qc
- fHV = 1 / (1 + (PHV) · (EHV − 1)) where PHV is a fraction.
- C = Clane · n · FU · fHV · Fped · Fm
- v = Ventry / PHF
- v/c = v / C
The coefficients A and B vary by country, geometry, driver behavior, and calibration dataset. Use your governing guide for those values.
How to use this calculator
- Measure or forecast the entry demand Ventry for the approach.
- Estimate the conflicting circulating flow Qc at that entry.
- Select a coefficient preset or enter A and B from your standard.
- Set the number of entry lanes and a realistic FU for lane usage.
- Apply adjustments for heavy vehicles, pedestrians, and any metering.
- Click Calculate to review capacity, v/c, reserve, and screening delay.
- Export a CSV or PDF report for checks, submittals, or internal review.
Professional notes and guidance
1) What this calculator represents
This roundabout capacity tool estimates the maximum sustainable entry flow for a single entry stream under a given conflicting circulating flow. It uses an exponential entry-capacity relationship with adjustment factors to reflect lane usage, heavy vehicles, pedestrian friction, and any upstream metering effects. Results are intended for screening, option comparisons, and documentation support during concept and preliminary design.
2) Understanding the core capacity curve
The base relationship Clane = A · e-B·Qc captures the declining ability of entering drivers to find acceptable gaps as circulating flow rises. For example, with A = 1130 and B = 0.0010, increasing Qc from 600 to 1200 veh/h reduces per-lane entry capacity from roughly 620 to 340 veh/h, before any additional factors are applied.
3) Typical planning inputs and data checks
Use measured turning counts, peak-hour approach volumes, and balanced design-year matrices when available. Keep units consistent in vehicles per hour. If your approach volumes are from a one-hour count, apply a realistic PHF (often 0.85–0.95) to capture short-term peaking. Validate circulating flows using the entry-to-entry interactions around the central island, not just a single arm count.
4) Heavy vehicles and operational friction
Heavy vehicles lower entry performance due to slower acceleration and longer gap requirements. The tool uses a passenger-car-equivalent adjustment where %HV and EHV define the reduction. As a reference, 8% HV with EHV = 2.0 yields fHV ≈ 0.926. Pedestrian activity near entries can further reduce effective capacity; apply Fped based on observed crossing demand and yield compliance.
5) Interpreting v/c, reserve, and reporting
The degree of saturation v/c is a robust screening indicator. Values below 0.85 typically suggest operational resilience, while values approaching 1.00 indicate sensitivity to minor fluctuations. Use reserve capacity (C − v) to compare design options and document whether added entry lanes, improved lane utilization, or reduced pedestrian conflict would yield the greatest benefit. Export CSV or PDF for review logs.
FAQs
1) Which values should I use for A and B?
Use coefficients published in your governing standard or from a locally calibrated model. Presets here are starting points only and should not replace jurisdictional requirements.
2) What is conflicting flow (Qc)?
It is the circulating stream flow that has priority over the entry. Higher conflicting flow reduces the number of acceptable gaps, lowering entry capacity.
3) Why do you adjust with PHF?
PHF converts an hourly volume into an equivalent peak flow rate for short-term surges. Lower PHF means sharper peaks and higher effective demand during the worst 15 minutes.
4) How should I pick lane utilization (FU)?
Choose based on observed lane balance and geometry. If one lane carries most traffic, FU drops below 1.00. Use a conservative value when uncertainty is high.
5) What does v/c greater than 1.0 mean?
It indicates oversaturation: demand exceeds the estimated capacity. Queues and delay can escalate quickly, and the simplified delay estimate is no longer reliable for screening.
6) How do heavy vehicles affect results?
They reduce capacity via the heavy-vehicle factor. Increase %HV or EHV when trucks and buses are frequent, steep grades exist, or acceleration is constrained.
7) Can I use this for final design submissions?
Use it for preliminary checks and comparisons. For final design, apply your jurisdiction’s full method, geometry checks, and any required software outputs or calibration steps.