You are almost done with the course project. At this point, all the fundamental pieces are in place. All that remains is to attach a simple user interface for driving the simulation and watching it run.
This simple user interface is not an elegant one, but it will work well enough to test the system. It would not be hard to pull this interface out and replace it with a better graphical interface.

Now it is time to finish the system you have been developing.
TrafficFlow System - Exercise

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems. Observer is also a key part in the familiar MVC architectural pattern. The observer pattern was first implemented in Smalltalk's MVC based user interface framework.