Q$-12$: An isolated three$-$phase signal is being designed for optimum cycle length of $90s.$ the total lost time

from all the three phases is $15s.$ The critical flow ratios for phase I and II are $0\mathrm{.}2$ and $0\mathrm{.}3$ respectively.

Determine the critical flow ratio for phase III as per Webster's method of traffic signal design. Given

that saturation flow for volume in phase III is $500$ pcu.

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