What is Traffic Signal Design?
Traffic signals are active control devices that assign right-of-way to competing traffic streams at intersections by displaying coloured light indications. Their design involves determining the optimal cycle length (how long one complete sequence of all phases takes) and the green splitting (how the available green time is distributed among the competing approaches).
Key Signal Terminology
| Term | Definition |
|---|---|
| Green time (G) | Time in each cycle for which a movement receives right-of-way |
| Amber/Yellow time (A) | Time in each cycle for clearance of intersection |
| Red time (R) | Time in each cycle for which movement stops |
| Cycle length (C) | Time for one complete rotation through all signal phases |
| Effective green (gᵢ) | Actual usable green = Gᵢ + yᵢ − t_L |
| Lost time (t_L) | Start-up lost time + clearance lost time per phase |
| Phase | Sum of green + amber + red times for a movement |
Webster Method — Optimum Cycle Length
Webster’s method designs the pre-timed signal cycle to minimise total delay at the intersection. The optimum cycle length is:
C₀ = (1.5L + 5) / (1 − Y)
Where: L = Total lost time per cycle = n × l + R (n = number of phases, l ≈ 4 sec per phase), Y = Sum of flow ratios = y_A + y_B + … (y_i = q_i / S_i = actual flow / saturation flow).
Green Splitting Formula
Once the optimum cycle length is found, the effective green time is distributed proportionally to the critical lane volumes:
Total effective green: T_g = C₀ − L
Green for phase i: gᵢ = (y_i / Y) × T_g
The critical lane volume for each phase is the volume in the lane with the most intense traffic — if that lane is adequately served, all other lanes in the same phase will also be adequately served.
Pedestrian Green Time
Green Pedestrian Interval (GPI) = Walk time + Clearance time
Green pedestrian time = 7 + W/1.2 (seconds)
Where W = width of road in metres, 7 sec = minimum walk time (start-up lost time for pedestrian), 1.2 m/s = pedestrian walking speed (15th percentile speed).
IRC Method
In the IRC method, signal timing is initially decided by the approximate method and then checked for both vehicular and pedestrian requirements. Key rules: each clearance amber and initial amber = 2 seconds. Cycle length = G_A + A_A + G_B + A_B. Minimum green time for vehicular traffic = 16 seconds. The cycle is rounded to the nearest higher multiple of 5 seconds.
Solved Example
Given: 2-phase signal, N-S and E-W roads. Design flows: N=1000, S=700, E=900, W=550 PCU/hr. Saturation flows: N-S=2500, E-W=3000 PCU/hr. Total lost time = 12 sec.
y_NS = 1000/2500 = 0.4 | y_EW = 900/3000 = 0.3 | Y = 0.7
C₀ = (1.5×12 + 5)/(1−0.7) = 23/0.3 = 77 seconds
T_g = 77 − 12 = 65 sec | g_NS = (0.4/0.7)×65 = 37 sec | g_EW = (0.3/0.7)×65 = 28 sec