Traffic Engineering
Signal Timing (HCM)
HCM signal timing calculator — Webster's optimal cycle length, phase splits, and critical v/c ratio for 2–4 phases.
Signal Timing — Webster's Method
Webster's method determines optimal cycle length and phase splits at isolated, pre-timed intersections by minimizing total vehicle delay. The optimal cycle is a function of the sum of critical volume-to-saturation-flow ratios (ΣYi) and total lost time per cycle (L).
The critical flow ratio for each phase (Yi = vi/si) is the demand volume divided by the saturation flow rate for the critical lane group. If ΣYi approaches 1.0 the intersection is near capacity; at or above 1.0 it is over capacity. Saturation flow is typically 1,600–1,900 vphg per lane, adjusted for lane width, grades, and turns.
C₀ = (1.5L + 5) / (1 − ΣYᵢ)
gᵢ = (Yᵢ / ΣYᵢ) × (C₀ − L)
Xc = ΣYᵢ × C₀ / (C₀ − L)
Signal Timing Calculator
Webster's method — isolated pre-timed intersection