For many of our experiments, we used networks with small traffic volumes, since the adaptive control logic seem to work better on these, rather than heavier volumes. The creature that is the focus of this demonstration is one that was fairly common. Here is the control law it used:
See the page "Using Demos" for important information.
A four intersection network was used when evolving this control law. It consisted of two one-way streets (both are west-bound) and two two-way streets (both north-south). The west-bound streets had a significantly higher volume, each having 300 cars/hour, and the north-south streets had 100 cars/hour.
As you can see from this control law, it is a fairly easy strategy for handling small traffic volumes. Whenever there is a car approaching in the stopped lane, it returns true, so that there will be a green light for the soon-to-be waiting car. The interesting part of this creature, though, is that whenever CONSTANT_1 is greater than CONSTANT_0, it returns true and switches phases. This might seem very strange at first, but it is actually intelligent behavior. If you examine the value of the constants, you will see that whenever the north-south streets are the moving streets (they have the green light), CONSTANT_1 is greater than CONSTANT_0 for all the intersections. However, whenever the eastbound streets are moving, CONSTANT_1 is not greater than CONSTANT_0 for all the intersections. In essence, whenever the north-south streets have a green light, the control law will immediately return true, so that the green light returns back to the eastbound traffic. Considering that the eastbound traffic is much heavier, this is very intelligent behavior. The control law was able to learn and adapt to the fact that the eastbound streets mattered move for reducing the traffic.
To sum up the control law, whenever a car approaches from the north or south, the signal switches phases so that the north south streets will receive one second of green time. Once that is over, it immediately returns the green to the busier eastbound traffic.
This creature was found after exploring 9400 different control laws, with a population size of 2000 creatures.