Traffic Signal Timing Calculator

Example data table

Formula used

• Adjusted demand: v_adj = v / (PHF · fHV)
• Critical flow ratio: y_i = v_adj,i / s_i
• Sum of ratios: Y = Σ y_i
• Total lost time: L = Σ(start‑up lost + yellow + all‑red)
• Optimized cycle: C = (1.5L + 5) / (1 − Y) (bounded by min/max)
• Usable green: G = C − L
• Green split: g_i = g_min,i + (G − Σg_min) · (y_i/Σy)
• Capacity: cap_i = s_i · (g_i/C)
• Degree of saturation: v/c = v_adj / cap
• Ped min green (if enabled): g_ped = walk + width/speed + buffer

Traffic demand and critical ratios

Enter volumes for the critical lane group in each phase. The calculator adjusts demand using Peak Hour Factor and a heavy‑vehicle factor, then computes y = v_adj / s. The sum (Y) shows how close operations are to saturation. As Y rises, an optimized cycle must grow to supply enough green for competing movements.

Lost time and clearance intervals

Start‑up lost time reflects driver reaction and queue start. Yellow and all‑red provide clearance, especially near temporary barriers and uneven surfaces. Total lost time L reduces usable green (G = C − L). If L is high, improve sight lines, markings, and detector placement to reduce hesitation and wasted seconds.

Green split strategy and minimums

After reserving minimum greens, remaining usable green is assigned in proportion to each phase’s y value. This prioritizes movements that drive delay while protecting minor phases. For coordinated corridors, use fixed cycle mode and tune splits to match progression. For agency minimums, raise the minimum green inputs accordingly. If minimums consume most of G, the calculator will flag it; increase cycle bounds or simplify phasing to regain usable green quickly onsite.

Pedestrian timing in work zones

For pedestrian phases, minimum green equals walk time plus crossing distance divided by walking speed, with an added buffer. Work zones often justify slower speeds and larger buffers because of detours, narrowed paths, and uneven grades. Measure distance from the temporary stop line to the temporary refuge, not the permanent curb width.

Interpreting v/c and field adjustments

v/c compares adjusted demand to computed capacity for each phase. Values near 1.00 suggest balance; values above 1.10 signal oversaturation and queues that may spill into the work area. Mitigate with turn restrictions, added channelization, revised phasing, higher saturation flow, or a longer cycle within safe limits. Recheck after any lane closure change.

FAQs

What does the saturation flow input represent?

It is the maximum hourly service rate for the critical lane group under ideal conditions. Use local guidance, lane width, grade, heavy vehicles, and work-zone constraints to adjust the value realistically.

Should I use optimized or fixed cycle mode?

Use optimized mode when you need a quick starting cycle for an isolated work-zone signal. Use fixed mode when coordination, agency policy, or preapproved timing sheets require a specified cycle length.

Why is my usable green time negative?

Usable green becomes negative when total lost time exceeds the cycle length. Reduce yellow or all-red only if standards allow, lower start-up lost time assumptions, or increase the cycle bounds to restore usable green.

How do pedestrian settings affect timing?

If pedestrians are enabled, the calculator enforces a minimum green based on walk time, crossing distance, walking speed, and buffer. This minimum can increase the cycle or reduce green available to other phases.

What v/c value is considered acceptable?

Many plans target v/c near 0.85 to 0.95 for stable queues. Values above 1.00 indicate growing queues, and above 1.10 often signals oversaturation that may require operational changes.

Can I use this output directly in the field?

Treat results as a planning baseline. Verify safety intervals, controller constraints, detection settings, and local standards. Perform short field observations and adjust splits or cycle length to match real queues.