How to Time a Traffic Signal — Step-by-Step Guide
Timing a pretimed (or the base plan of an actuated) signal follows a fixed sequence: convert counts to flow rates, pick phasing, set change intervals, size the cycle, split the green, and check the result. This guide follows the three-phase example in the free signal timing calculator: a 45 mi/h approach, 60-ft crossing and critical flows of 650, 420 and 250 veh/h.
Convert counts to peak flow rates
Divide by the number of lanes in the lane group to get per-lane flow. Shared lanes and turning traffic need the HCM's adjustments; for planning, convert left turns to through-vehicle equivalents.
Choose phasing and find critical lane groups
Start with two phases. Add a protected left-turn phase when left-turn volume, opposing volume or crash history requires it (a common screen is a cross-product of left turns × opposing through over about 50,000–100,000 per hour, but agencies differ). Every added phase adds lost time, so do not add phases casually. For each phase, compute y = v/s for each lane group and keep the largest.
Set yellow and all-red
AR = (W + L)/1.467v = (60 + 20)/66.0 = 1.2 s
Set pedestrian intervals
Total the lost time and pick a cycle length
Webster: Co = (1.5L + 5)/(1 − ΣY) = 23/0.258 = 89.0 s → C = 90 s
HCM minimum at Xc = 0.90: Cmin = L Xc/(Xc − ΣY) = 68 s
Split the green
g1 = (0.361/0.742)(78) = 38.0 s · g2 = 24.5 s · g3 = 15.5 s
Displayed G = g − Y − AR + tL → 36.5, 23.0 and 14.0 s
Check v/c, delay, minimum greens and pedestrians
- v/c: Xc = ΣY × C/(C − L) = 0.742 × 90/78 = 0.856 for every critical group. Keep it under about 0.90.
- Delay: uniform delay is 23.5, 31.0 and 36.2 s/veh by phase. Add random delay (HCM d2) for the full control delay and level of service.
- Minimum green: typically 5–7 s for driver expectancy, more on high-speed approaches.
- Pedestrians: phase 3 provides 14.0 + 4.3 + 1.2 = 19.5 s, less than 24.1 s. Lengthen it, move the crosswalk to a longer phase, or give it a pedestrian-actuated extension.
Fine-tune in the field or in simulation
Watch for phases that gap out early (too much green), queues that don't clear (too little), and left-turn bays that overflow. The Stopbar simulator lets you sketch the intersection and watch queues under fixed or actuated timing before going to the cabinet.
Yellow, all-red, ped timing, Webster and HCM cycles, splits, v/c and delay for up to four phases.
Open the signal timing calculator →Worked examples
Four-phase signal with protected lefts · Intersection crash rate screening · Freeway incident shockwave
Frequently asked questions
What is a critical lane group?
In each phase, the lane group with the highest flow ratio v/s. It needs the most green, so it sets the phase length; other lane groups moving in the same phase get spare capacity. The sum of the critical ratios, ΣY, measures how loaded the intersection is.
What saturation flow should I use?
The HCM starts from 1,900 passenger cars per hour of green per lane and applies adjustment factors for lane width, heavy vehicles, grade, parking, bus blockage, area type and turning movements. Typical adjusted through-lane values are 1,700–1,850 veh/h/ln; protected lefts are a little lower.
Why convert counts with the peak hour factor?
Signals must serve the busiest 15 minutes, not the hourly average. Dividing hourly volume by the PHF gives the peak 15-minute flow rate in veh/h, which is what v in v/s should be.
What is the difference between effective green and displayed green?
Effective green is the time traffic actually flows at saturation. Displayed green is what the signal head shows. They are related by g = G + Y + AR − tL: vehicles keep moving into the yellow, but the first seconds of green and the end of the change interval are lost.