An additional problem arises when two roads cross. Now one Place object that is at the point of

intersection will belong to two Road objects. Because in C$++$ we need to do our own garbage

collection, writing a destructor for this linked list requires the use of a reference count. We can

only delete Place objects that are not parts of any other Road object.

The TrafficLight object has several interesting features. First, we need to represent the current

status of the light and define the transitions between the possible states. Our traffic light design

requires that the signal in the two opposite directions $($north$/$south or east$/$west$)$ are the same at all

times. There are at least two methods for recording the current state and computing the new state

on each timer update. The first method requires that we store the timing data supplied by the user

and every time the light color changes we start a countdown with the appropriate value. The design

works, but the implementation is quite messy.