Work out roundabout capacity instantly with clear inputs, formula shown and shareable results.
Empirical roundabout models express entry capacity as a linear function of circulating flow: capacity falls by about 0.7 vehicles for each additional circulating vehicle. The intercept, capacity with no circulating traffic, rises with entry width, flare length and inscribed circle diameter, which is why widening the entry is the most effective geometric improvement.
Linear entry capacity
Qe = intercept - 0.7 x circulating flow
Intercept
Rises with entry width, effective flare length and inscribed circle diameter
Preliminary estimate only. Roundabout capacity should be assessed with the empirical model and geometric parameters required by the road authority.
Qe = intercept - 0.7 x circulating flow. Empirical roundabout models express entry capacity as a linear function of circulating flow: capacity falls by about 0.7 vehicles for each additional circulating vehicle.
Entry width determines how many vehicles can accept a gap simultaneously. A larger circle mainly increases speed, which is often undesirable, while a flared entry directly increases throughput.
This calculator takes 4 inputs: Entry width, Circulating flow across the entry, Average effective flare length, Inscribed circle diameter. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.