Highway Engineering
A traffic rotary is justified where

when traffic volume is more than 5000 vehicles per hour
space is limited and costly
when traffic volume is less than 500 vehicles per hour
number of intersecting roads is between 4 and 7

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Highway Engineering
When the speed of traffic flow becomes zero,then

traffic density attains maximum value whereas traffic volume becomes zero
traffic density and traffic volume both become zero
traffic density and traffic volume both attain maximum value
traffic density becomes zero whereas traffic volume attains maximum value

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