This simple model illustrates the behaviour of traffic.
A circular road is depicted as a series of 100 spaces, each with room for one car, the cars proceed according to the (simultaneously applied) simple rules:
Try 45 cars. Look at the applet - what happens is immediately clear, traffic jams form (red cars), move backwards along the road, and eventually coalesce. Now, you can study the size and velocity of the traffic jams as a function of number of cars.
The cars are moving anticlockwise on the road; you can change the probability that a stationary car moves off if there is a free space ahead.