Why a flyover

At a busy junction, crossing traffic streams have to take turns, and that waiting is what causes queues. A flyover separates the streams vertically so the main flow can pass without stopping. This is called grade separation.

A flyover is a bridge built over a road rather than a river. In cities it is usually a viaduct of piers and precast girders or segments, built quickly to limit disruption to the traffic below. Ramps at each end bring vehicles up and down at a gradient that heavy vehicles can manage.

A flyover works only if the traffic it delivers can be absorbed at the next junction; otherwise the queue simply moves. So planners look at the corridor as a whole, alongside signal improvements, public transport and pedestrian crossings.

Where many movements must be separated, as at highway junctions, flyovers combine with loops and slip roads into interchanges such as trumpets, cloverleafs and diamonds.

Designing a flyover also means designing for the street below. Pier positions must leave room for turning vehicles, pedestrians and drainage. Clearance under the deck must suit buses and trucks. Construction staging decides how many lanes stay open while piers and girders go up, and precast segments or girders lifted at night help keep the city moving. Lighting, noise and the look of the structure matter to the people who live beside it for the next hundred years.