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Guesstimate Example # 9 | How many signals are red at this particular moment in Kolkata?


Q. How many signals are red at this particular moment in Kolkata?

Assuming average peak city hour when all signals are functioning normally

Traffic Signal color duration:
Red – 45% of the time
Green – 45% of the time
Yellow – 10% of the time

Area of Delhi = 1400 sq km (known by knowledge)

Area of Kolkata = 300 sq km = 3×10^8 sq m (assumed by comparison)

By Govt. suggested City Planning, 20% area of a city should be roads

Total area of roads = 0.2x3x10^8 = 6×10^7 sq m

Width of road = 40m

Length of roads = [6×10^7 sq m]/40m = 2×10^6 m

A signal is present every 400m

Total no. of signals in Kolkata = [2×10^6 m]/400m = 5,000

Total no. of signals Red at any point of time:

0.45×5000 = 2250 = rounded to= 2500 signals

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