Method and limits

Cycle length is Webster's optimum, C₀ = (1.5L + 5) / (1 − Y), where L is total lost time (lost time per phase × number of phases, plus all-red) and Y = Σ(q/s) is the sum of critical flow ratios. Effective green is apportioned as gᵢ = (C − L) · yᵢ / Y. Delay uses the first two terms of Webster's equation — uniform delay plus random-arrival overflow delay — which is the standard approximation and omits the small third correction term.

Webster's method assumes isolated, undersaturated, random arrivals. It does not model coordination with adjacent signals, platooned arrivals, pedestrian stage minimums, permitted-protected left turns, short-lane blocking, or actuated control. Where Y approaches 1.0 the formula diverges and the result is meaningless; the tool refuses to produce a plan above Y = 0.95 and tells you so. For a corridor, these numbers are a starting point for a microsimulation model, not a substitute for one.

Saturation flow should be a measured value where you have one. The 1,800 pcu/h/lane default is an ideal figure before adjustment for lane width, gradient, heavy vehicles, parking, and turning movements — real urban approaches commonly fall between 1,500 and 1,750.