Jams and superjams: a systems approach to congested traffic networks

The road traffic engineer attempts to solve the problem of congestion and delay to road traffic in urban road networks by increasing capacity. He does this by computer control of traffic signals, by traffic management (one-way systems, banned turns etc.), by junction design, and to a limited extent by building new or improved roads. Such increases in capacity do not increase traffic speed if traffic demand is sufficiently high. However, they do increase the risk of traffic jams when blockages of the network, and especially of junctions occur, because there is increased traffic on essentially the same network as before. Traffic speed in congested road networks is set by an equilibrium with the best alternative public transport system. It can only be improved if public transport is improved and road space devoted to the more efficient user of space, the bus or tram, or, if demand is sufficiently high, by the construction of segregated rail systems.