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Saturation Flow Rate Calculator

Work out saturation flow rate instantly with clear inputs, formula shown and shareable results.

About Saturation Flow Rate Calculator

Saturation flow is the maximum discharge rate from a signal stop line once the queue is moving, about 1900 PCU per hour per lane under ideal conditions. Adjustment factors reduce it for narrow lanes, uphill approaches and turning movements, which take longer to clear because drivers must judge gaps or negotiate a smaller radius.

Formula

Adjusted saturation flow

s = base saturation x width factor x gradient factor x turning factor

Worked example

With the default values below, Saturation Flow Rate Calculator returns Adjusted saturation flow of 1,797. Change any input above and every figure here is recalculated in your browser.

Inputs used

Base saturation flow per laneminimum 500 PCU/h
1,900 PCU/h
Lane widthminimum 2 m
3.5 m
Approach gradientaccepts -10–10 %
2 %
Share of turning movementsaccepts 0–100 %
10 %

Results

Adjusted saturation flow
1,797 PCU/h
Lane width factor
1.0000
Gradient factor
0.9600
Turning movement factor
0.9850

Step by step

  1. 1Values used
  2. 2Adjusted saturation flow
  3. 3Adjusted saturation flow= 1,797 PCU/h
  4. 4Lane width factor= 1.0000
  5. 5Gradient factor= 0.9600
  6. 6Turning movement factor= 0.9850

Assumptions and limits

  • The result is derived only from the 4 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 4 decimal places, 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 Saturation Flow Rate figure worked out?

s = base saturation x width factor x gradient factor x turning factor. Saturation flow is the maximum discharge rate from a signal stop line once the queue is moving, about 1900 PCU per hour per lane under ideal conditions.

Should saturation flow be measured rather than estimated?

Yes where possible. Measured discharge headways at the site are far more reliable than factored defaults, because local driver behaviour dominates.

What values do I need to enter?

This calculator takes 4 inputs: Base saturation flow per lane, Lane width, Approach gradient, Share of turning movements. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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