[reuven]

A model of this is included in the NetLogo models library, which comes with the NetLogo modeling language: http://ccl.northwestern.edu/netlogo/models/TrafficBasic

It's fun to see children (and adults!) play with the controls on this model, experimenting with different factors that they think might eventually reduce the congestion.

This model is also a great way to introduce kids to complex systems, in which the large-scale results aren't necessarily as obvious as the individual actions that lead to those results.