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Road Capacity Per Lane Calculator

Work out road capacity per lane instantly with clear inputs, formula shown and shareable results.

About Road Capacity Per Lane Calculator

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.

Formula

Greenshields capacity

Capacity = free flow speed x jam density / 4, at half of each

Worked example

With the default values below, Road Capacity Per Lane Calculator returns Capacity per lane of 2,600. Change any input above and every figure here is recalculated in your browser.

Inputs used

Free flow speedminimum 5 km/h
80 km/h
Jam densityminimum 10 veh/km/lane
130 veh/km/lane
Number of lanesminimum 1
2

Results

Capacity per lane
2,600 veh/h/lane
Total directional capacity
5,200 veh/h
Speed at capacity
40.0 km/h
Density at capacity
65.0 veh/km

Step by step

  1. 1Values used
  2. 2Greenshields capacity
  3. 3Capacity per lane= 2,600 veh/h/lane
  4. 4Total directional capacity= 5,200 veh/h
  5. 5Speed at capacity= 40.0 km/h
  6. 6Density at capacity= 65.0 veh/km

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Road Capacity Per Lane figure worked out?

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.

Is Greenshields realistic?

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.

What values do I need to enter?

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.

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