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

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Highway Engineering
Centre line markings are used

to designate traffic lanes
to designate proper lateral placement of vehicles before turning to different directions
in roadways meant for two way traffic
to indicate that overtaking is not permitted

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