The 'Webster''s formula' for optimum cycle length (Co) of a signalised intersection gives:
Choose the correct answer
Co = 60 seconds always
Co = (1.5L+5)/(1−Y) — balances total lost time against capacity utilisation; minimises average vehicle delay
Co depends only on number of lanes
Co = sum of all phase green times
Correct Answer
B. Co = (1.5L+5)/(1−Y) — balances total lost time against capacity utilisation; minimises average vehicle delay
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Webster (1958): Co = (1.5L+5)/(1-Y) where L = total lost time per cycle (= sum of all phase lost times), Y = sum of critical lane volume ratios (Σyi = qi/si for critical phases). Optimum Co minimises total delay. If Y → 1: Co → infinity (oversaturation). Degree of saturation x = yC/g must be ≤ 0.9 for each phase. Used for signal timing design.
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