  Traffic congestion is usually observed at the upper streams of bottlenecks
such as tunnels. Congestion appears as stop-and-go waves and high density
uniform flow. We perform simulations of traffic flow with a bottleneck using
the coupled map optimal velocity model. The bottleneck is expressed as a road
segment with speed reduction. The speed reduction in the bottleneck controls
the emergence of stop-and-go waves. A phenomenological theory of bottleneck
effects is constructed.
