Traffic basics

Traffic engineering starts with three measurable quantities. Flow is vehicles passing a point per hour. Density is vehicles per kilometre of lane at an instant. Speed links them: flow equals density times speed.

Speed falls as density rises. At low density drivers travel freely; as vehicles crowd together they slow down. Flow rises with density up to a maximum, the capacity, then falls as traffic becomes congested. The simplest model, by Greenshields, assumes speed drops in a straight line with density, which gives a parabola for flow and a capacity at half the jam density.

Indian traffic is mixed: two-wheelers, autos, cars, buses and trucks share lanes. Counts are converted to passenger car units so different vehicles can be compared.

These ideas feed the design of lanes, intersections and signals, and they explain why a small loss of capacity, a stalled bus or a lane closure, causes long queues at busy times.