Pavements & traffic · Traffic & intersections
Roundabout entry capacity and delay calculator
Each approach enters against the circulating flow in front of it. Entry capacity follows Tanner's gap-acceptance formula with the critical gap, follow-up headway and minimum circulating headway (scaled for a second entry lane); the degree of saturation, control delay and 95th-percentile queue follow, with the level of service for an unsignalised intersection.
Standard: Tanner (1962) gap-acceptance capacity as used in Austroads Guide to Traffic Management Part 3 · HCM 2010 roundabout control delay
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Approaches: name, entry flow (veh/h), circulating flow (veh/h), entry lanes | North, 620, 480, 1 East, 540, 560, 1 South, 700, 390, 2 West, 380, 650, 1 | |
| Critical gap α | 4 | s |
| Follow-up headway β | 2.5 | s |
| Minimum circulating headway Δ | 2 | s |
| Second entry lane factor | 0.8 | |
| Analysis period | 0.25 | h |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| North: capacity 951 veh/h · x 0.65 | delay 13.8 s · 95th queue 5.0 veh · LOS B | |
| East: capacity 877 veh/h · x 0.62 | delay 13.5 s · 95th queue 4.4 veh · LOS B | |
| South: capacity 1,866 veh/h · x 0.38 | delay 5.0 s · 95th queue 1.8 veh · LOS A | |
| West: capacity 797 veh/h · x 0.48 | delay 11.0 s · 95th queue 2.6 veh · LOS B | |
| Average delay (entry-flow weighted) | 10.5 | s |
| Roundabout level of service | B |
- ✓ Every approach x ≤ 0.85 (practical) — 0.65
- ✓ No approach over capacity
Method, formulas and assumptions
- Tanner: Qe = 3600·qc(1 − Δqc)·e^(−qc(α − Δ))/(1 − e^(−qc·β)) with qc in veh/s; a second entry lane adds the stated factor of the single-lane capacity.
- Control delay (HCM 2010): d = 3600/c + 900T[x − 1 + √((x − 1)² + (3600/c)x/(450T))] + 5·min(x, 1); 95th queue from the same form with 150T.
- Typical Australian values: α = 3.5–4.5 s, β = 2.0–2.6 s, Δ = 2 s (single circulating lane). Calibrate with SIDRA for design.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.