The lights use a wireless radio at 900MHz or 5.8GHz to transmit data to each other. They are all on the same subnet. Entering the network doesn’t require a password and the data is unencrypted. The controller for a network has a debug port opened by default. It’s thus easy to get into the controller and send your own commands. Then you can change lights and control cameras!

Researchers find it’s terrifyingly easy to hack traffic lights | Ars Technica.