This project models the movement of cars on a highway. It demonstrates how traffic jams can form simply as the result of how each driver reacts to the car in front, even without any accidents or constrictions in the road. No "centralized cause" is needed for a traffic jam to form.
For detailed instructions on using the program, take a look at the info window from the Traffic Basic model.
Click on one of the pictures to see a quicktime movie of the model: