Roundabout Capacity Calculator
Input Parameters
Traffic Flow Parameters
Lane Configuration
Conflicting Flow Parameters *
Calculation Results
Capacity Analysis Summary
Step-by-Step Calculations
Real-World Implications
Engineering Interpretation
Practical Recommendations
Roundabout Visualization
What is Roundabout Capacity Calculator?
Roundabout entry capacity is the maximum sustainable flow rate (pc/h) that can enter a roundabout from one approach under prevailing circulating flow, based on gap-acceptance theory.
The Roundabout Capacity Calculator for Highway/Civil Engineers is a fast and accurate online tool that instantly computes entry capacity, degree of saturation, control delay, and 95th-percentile queue for single-lane and multi-lane roundabouts using HCM 2016 (HCM6) and HCM 2010 methods. Perfect for roundabout capacity calculator, HCM6 roundabout entry capacity, single-lane vs multi-lane roundabout design, gap-acceptance analysis, and quick intersection capacity checks.
This roundabout capacity calculator offers clear visualizations, a dedicated space for comments, analysis, and recommendations, a complete step-by-step calculation workflow, CSV export of results, and a colorblind-friendly view to enhance accessibility.
How to use Roundabout Capacity Calculator?
Purpose: Determine how much traffic a roundabout can handle before it becomes congested, so you can size the roundabout correctly and check LOS.
Inputs you will enter:
- Entry flow rate v_e (pc/h)
- Circulating/conflicting flow rate v_c (pc/h)
- Lane configuration (1×1, 1×2, 2×1, 2×2, etc.)
- HCM version (2010 or 2016)
- Optional: heavy vehicle %, pedestrian flow, bypass lane
Roundabout Capacity Formula
\(c_e = A \times \exp(-B \times v_c)\)
Where (HCM6 single-lane): \(A = 1425,\ B = 0.00085\)
Where (HCM6 2×2): Right lane: \(A = 1425,\ B = 0.00085\) Left lane: \(A = 1430,\ B = 0.00070\)
How to Calculate Roundabout Capacity (Step-by-Step)
- Enter entry demand v_e and circulating flow v_c.
- Select lane configuration and HCM version.
- Calculator applies the correct A and B values.
- Compute capacity c_e per lane (then sum for total).
- Adjust for heavy vehicles and pedestrians if needed.
- Calculate degree of saturation x = v_e / c_e, delay, and queue.
- Review recommendations (LOS, storage length, etc.).
Examples
Example 1 – Single-Lane Roundabout (HCM6) Entry demand v_e = 650 pc/h Circulating flow v_c = 900 pc/h \(c_e = 1425 \times \exp(-0.00085 \times 900) \approx 1425 \times 0.466 \approx 664\ \text{pc/h}\) x = 650 / 664 ≈ 0.98 → near capacity, LOS E
Example 2 – Two-Lane Roundabout (HCM6 2×2) v_e = 1,200 pc/h (600 per lane) v_c = 1,600 pc/h Right lane: \(c_e = 1425 \times \exp(-0.00085 \times 1600) \approx 1425 \times 0.257 \approx 366\ \text{pc/h}\) Left lane: \(c_e = 1430 \times \exp(-0.00070 \times 1600) \approx 1430 \times 0.326 \approx 466\ \text{pc/h}\) Total c_e ≈ 832 pc/h x = 1,200 / 832 ≈ 1.44 → oversaturated, LOS F
Roundabout Capacity Categories / Normal Range
| Configuration | HCM Version | Typical Capacity (pc/h) | Max Practical v_c (pc/h) | Recommended Max x |
|---|---|---|---|---|
| Single-lane (1×1) | HCM6 | 650–1,400 | 1,200 | 0.85 |
| Single-lane (1×1) | HCM2010 | 550–1,200 | 1,000 | 0.85 |
| Two-lane (2×2) | HCM6 | 1,600–2,200 | 2,400 | 0.85 |
| Two-lane entry (2×1) | HCM6 | 1,800–2,400 | 1,800 | 0.85 |
| Three-lane | HCM6 ext. | 2,400–3,200 | 3,200 | 0.85 |
Limitations
- Empirical gap-acceptance model; does not replace microsimulation (SIDRA, VISSIM).
- Valid for balanced circulating flows; unbalanced flows reduce accuracy.
- HCM6 capacities are ~30–50% higher than HCM2010.
- Pedestrian and heavy vehicle adjustments are approximate.
- Does not check geometric constraints, sight distance, or safety.
Disclaimer
This calculator is provided for educational purposes, learning, and preliminary design checks only. All final roundabout designs must be verified with approved software and reviewed by a qualified professional traffic/highway engineer. The developer and platform are not liable for any errors, misinterpretations, or consequences arising from the use of these results in actual construction projects.