What is Webster's method?

Webster's method (1958) is a classical traffic engineering formula that minimizes total vehicle delay at a signalized intersection. It derives an optimal cycle length based on the total lost time per cycle and the sum of critical flow ratios (Y).

What is the saturation flow rate?

Saturation flow rate (s) is the maximum number of vehicles that can pass through a lane during one hour of continuous green. It typically ranges from 1,500 to 1,900 vehicles per hour of green (vphg) for through lanes under standard conditions.

What is lost time per phase?

Lost time accounts for start-up delay when the light turns green and clearance time at the end of the phase. A common value is 4 seconds per phase (2 s start-up + 2 s clearance). This tool uses 4 s/phase by default.

Why is Y capped at 0.9?

If the sum of critical flow ratios (Y) approaches or exceeds 1.0, the intersection is effectively over-saturated and no fixed-time cycle can clear the demand. The formula breaks down above Y ≈ 0.9; the calculator warns when this occurs.

How is green time split between phases?

After subtracting total lost time from the cycle, the remaining effective green time is distributed proportionally to each phase's flow ratio (vi/si) relative to the sum Y.

Does this include pedestrian minimums?

This calculator focuses on vehicular timing only. Always check that computed green times meet the minimum pedestrian crossing time for your intersection before finalizing a timing plan.

Traffic Signal Timing Calculator

Calculate a preliminary fixed-time traffic-light cycle with Webster's equation, using separate critical demand and saturation flow for each phase.

Intersection Parameters

Number of Phases

Lost Time per Phase (seconds)

Typical range: 3–5 s. Default is 4 s/phase.

Default Saturation Flow Rate (veh/hr/lane)

Used when phase rows are created; each row remains editable.

PhaseCritical demandSaturation flow

Enter intersection parameters and click Calculate Timing to see results.

Optimal Cycle

—

seconds

Total Lost Time

—

seconds

Sum Y (v/s)

—

flow ratio

Phase Timing Breakdown

Phase	Volume (veh/hr)	Sat. Flow	v/s Ratio	Green (s)	% of Cycle

Cycle Allocation

Formula: Co = (1.5L + 5) / (1 − Y), where L = total lost time, Y = sum of critical flow ratios.

The raw Webster result is rounded up to the next 5-second increment; no arbitrary minimum or maximum is silently imposed.

Inputs must represent the critical lane or lane group for each phase. Results are for an isolated, fixed-time, undersaturated planning case; verify pedestrian minimums, clearance, coordination and local standards.

Summary

Calculate a preliminary fixed-time traffic-light cycle with Webster's equation, using separate critical demand and saturation flow for each phase.

How it works

Enter the number of signal phases (2–6) for your intersection.
Set a default saturation flow, then edit the critical-lane saturation flow for each phase where needed.
Enter the critical-lane demand for each phase in vehicles per hour.
The calculator computes the flow ratio (v/s) and critical-lane volume for each phase.
Webster's optimal cycle length formula (Co = (1.5L + 5) / (1 - Y)) is applied.
Green time is distributed proportionally to each phase's flow ratio.

Use cases

Preliminary signal timing design for new intersections.
Re-timing existing signals to reduce delay and improve level of service.
Transportation engineering coursework and exam preparation.
Evaluating signal efficiency for traffic impact studies.
Comparing timing plans for peak vs. off-peak periods.
Quick feasibility checks before full simulation software runs.

Frequently Asked Questions

Last updated: 2026-09-23 · Reviewed by Nham Vu