Work out road capacity per lane instantly with clear inputs, formula shown and shareable results.
The Greenshields linear speed density model gives capacity as free flow speed times jam density divided by four, occurring at half the free flow speed and half the jam density. It explains why traffic breaks down at moderate speeds: maximum throughput happens well below the speed limit, and pushing past it reduces flow.
Greenshields capacity
Capacity = free flow speed x jam density / 4, at half of each
Capacity = free flow speed x jam density / 4, at half of each. The Greenshields linear speed density model gives capacity as free flow speed times jam density divided by four, occurring at half the free flow speed and half the jam density.
It is a simplification, and observed capacity typically occurs at a higher speed than half the free flow value. It remains the clearest way to show that flow, not speed, is what a road maximises.
This calculator takes 3 inputs: Free flow speed, Jam density, Number of lanes. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.