Calculator
Example data table
These sample rows illustrate typical inputs and resulting outputs.
| Scenario | Lanes | Cycle (s) | Green (s) | Width (m) | HV % | Grade % | Area | PHF | Dir share | Design cap (veh/h) |
|---|---|---|---|---|---|---|---|---|---|---|
| Collector street | 1 | 80 | 28 | 3.3 | 6 | 1 | URBAN | 0.92 | 0.55 | 323 |
| Arterial approach | 2 | 100 | 40 | 3.6 | 10 | 2 | CBD | 0.9 | 0.6 | 724 |
| Suburban corridor | 3 | 90 | 38 | 3.5 | 5 | -1 | SUBURBAN | 0.95 | 0.55 | 1,270 |
Formula used
This tool uses a signalized approach capacity model with adjustable saturation flow.
- Adjusted saturation flow: s = s0 × f
w × f HV × f g × f a - Green ratio: g/C
- Capacity per lane: clane = s × (g/C)
- Capacity per direction: cdir = clane × lanes
- Design-direction capacity: cdd = cdir × directional share
- Effective service flow: SF = cdd × PHF
- Volume-to-capacity: v/c = demand ÷ cdd (when demand is provided)
The adjustment factors are simplified engineering approximations. For final design, calibrate inputs with local standards, turning flows, and observed discharge rates.
How to use this calculator
- Enter lanes per direction for the segment or approach.
- Add cycle length and effective green from signal timing plans.
- Set lane width, heavy vehicle share, and roadway grade.
- Choose an area type to reflect access density and friction.
- Provide PHF and design-direction share for peak conditions.
- Optionally enter demand to get v/c and LOS.
- Click Calculate Capacity, then export CSV or PDF.
If your street is unsignalized, use a different method. This model is intended for signal-controlled movements where green time governs discharge.
Understanding street capacity at signal control
Street capacity on urban corridors is commonly limited by signal timing and discharge behavior at stop lines. This calculator estimates directional capacity using an adjusted saturation flow rate and the effective green ratio. It is useful for screening alternatives, checking whether queues are likely, and documenting assumptions consistently.
Key inputs that drive the result
The cycle length and effective green time set the share of the hour available to serve traffic. Lanes per direction scale the served volume, while lane width, grade, and area friction modify discharge rates. Heavy vehicles reduce flow because larger headways and acceleration limits lower the effective saturation flow.
Interpreting output for design decisions
Capacity per lane and capacity per direction describe the supply under prevailing timing. When demand is entered, the calculator returns a v/c ratio and a simple LOS letter for quick communication. Values near or above 1.00 indicate unstable operation, rising delay, and a high risk of spillback.
Example data for a quick check
Sample scenario and computed design-direction capacity.
| Scenario | Lanes | Cycle | Green | HV % | Grade % | PHF | Dir share | Design cap |
|---|---|---|---|---|---|---|---|---|
| Urban collector | 1 | 80 s | 28 s | 6 | +1 | 0.92 | 0.55 | ≈ 305 |
Use your local base saturation flow and confirmed green time to refine results.
Practical calibration and limits
For stronger accuracy, calibrate base saturation flow with field observations, and ensure effective green reflects lost time. Where turning movements, parking friction, bus stops, or access density dominate, apply conservative factors or model by movement. For final design, pair these estimates with detailed intersection analysis and queue checks.
FAQs
1) What is “effective green” and why does it matter?
Effective green is the usable portion of green after lost time. It directly controls the g/C ratio, so even small changes can shift capacity meaningfully.
2) Why does heavy vehicle percentage reduce capacity?
Trucks and buses usually accelerate slower and maintain larger headways. That lowers the discharge rate, so the adjusted saturation flow declines as heavy vehicle share increases.
3) Should I enter total corridor demand or approach demand?
Enter demand for the direction or movement you are evaluating. If you are screening a corridor, use the peak direction flow at the critical intersection or bottleneck.
4) How do I choose peak hour factor (PHF)?
PHF reflects within-hour peaking. Use measured traffic counts when available. If unknown, 0.88–0.95 is common for many urban streets, with lower values indicating sharper peaks.
5) What does v/c above 1.0 mean here?
It indicates demand exceeds estimated capacity for the evaluated direction. Expect growing queues, higher delay, and possible spillback unless timing, lanes, or demand patterns change.
6) Is this suitable for unsignalized streets?
No. Unsignalized capacity is governed by gap acceptance and conflicting flows. Use an unsignalized intersection method or movement-based analysis for those conditions.
7) How can I improve accuracy for final design?
Use observed discharge rates, confirm lost times, model turning movements explicitly, and compare with detailed intersection tools. Always verify storage and queue lengths for critical approaches.