Why do we need signals?

To avoid chaos and gridlocks, we need to regulate traffic. Rotaries are space sharing devices where traffic from each direction merges, goes around the circle and weaves out. However, rotaries cannot function when PCU (a measure of vehicle density) exceeds 5000. Also rotaries require better discipline and awareness of rules regarding right of way on the part of road users.
Beyond 5000 PCU we require signals which are time sharing devices; allowing one stream of traffic at a time to avoid any merging, weaving and conflict. However, beyond 10,000 PCU, even signals become unmanageable due to abnormal increase in cycle time. We can still continue up to 12000 PCU by restricting few turning movements. Beyond 12000 PCU a grade separator is compulsorily required. Grade separators share space vertically and are the best solution for handling a junction with high PCU
Signals are essential part of traffic management in any city. It is neither feasible nor desirable to make grade separator at each junction. Essentially signals bring discipline. Moreover for the pedestrians these are the safest places for crossing the roads.