Enter Signal Timing Inputs
Use active phases only. Extra phase fields may stay unused.
Formula Used
This calculator uses the Webster cycle length method for signalized intersections and temporary construction traffic control.
Critical flow ratio for each phase: yi = vi / si
Sum of critical ratios: Y = Σyi
Total lost time per cycle: L = Σ(startup lost time + clearance lost time)
Webster optimum cycle: C = (1.5L + 5) / (1 - Y)
Green split per phase: gi = (yi / Y) × (Cadopted - L)
After finding the Webster cycle, the tool applies your minimum cycle, maximum cycle, and rounding step.
It then distributes effective green time in proportion to each phase demand.
How to Use This Calculator
- Enter the project or intersection name.
- Choose the number of active phases.
- Set your minimum cycle, maximum cycle, and rounding step.
- For each active phase, enter critical lane volume and saturation flow.
- Add startup lost time and clearance lost time for every active phase.
- Click the calculate button to view the adopted cycle and green splits.
- Review the table, capacity values, and Plotly graph.
- Download the results as CSV or PDF for field records.
Example Data Table
Sample input set for a four-phase work zone intersection.
| Phase | Movement | Volume (veh/h) | Saturation (veh/h) | Startup Lost (s) | Clearance Lost (s) | Critical Ratio |
|---|---|---|---|---|---|---|
| Phase 1 | Northbound Through | 300 | 1800 | 2 | 2 | 0.167 |
| Phase 2 | Southbound Through | 360 | 1800 | 2 | 2 | 0.200 |
| Phase 3 | Eastbound Approach | 250 | 1800 | 2 | 2 | 0.139 |
| Phase 4 | Westbound Approach | 220 | 1800 | 2 | 2 | 0.122 |
Example summary:
- Total lost time = 16 seconds
- Sum of critical ratios = 0.628
- Webster cycle ≈ 77.92 seconds
- Rounded adopted cycle = 80 seconds
FAQs
1. What is signal cycle length?
Signal cycle length is the total time needed for all signal phases to run once. It includes green time, change intervals, and all lost time in one complete cycle.
2. What method does this calculator use?
It uses the Webster cycle length equation. This method estimates an efficient cycle from total lost time and the sum of critical flow ratios for the active phases.
3. What is lost time in a signal cycle?
Lost time is the portion of the cycle not used for effective discharge. It usually includes startup delay and clearance intervals such as amber and all-red time.
4. Why must the critical ratio sum stay below 1.00?
A value of 1.00 or more means demand equals or exceeds available capacity. In that case, Webster timing stops being reliable and queues will usually keep growing.
5. Why does the calculator round the cycle length?
Field timing plans are commonly rounded to practical increments. Rounding makes plans easier to implement, review, coordinate, and communicate to site teams.
6. Can I use this for construction detours or temporary traffic control?
Yes. It is suitable for preliminary planning at temporary signalized work zones or detour intersections. Final settings should still be checked against field observations and local standards.
7. What saturation flow should I enter?
Enter the expected saturation flow for the critical lane group of each phase. Use a locally accepted value or a site-specific estimate based on lane width, grade, and heavy vehicles.
8. Why are green times split by phase ratio?
Proportional green splits give more effective green to phases with higher demand. This keeps service balanced and helps reduce uneven delay across movements.