The Christchurch City Council is upgrading their co-ordinated traffic signal control system to allow a more flexible method of controlling urban traffic flows. Essentially the upgrade consists of replacing the existing master controller with a minicomputer data concentrator, and the introduction of a microprocessor based devices to interface between the data concentrator and the existing signal installation hardware. Figure 1.1 shows the existing traffic control system. This project deals with one specific aspect of the upgrade, namely the design of the protocol for the communication links between the data concentrator and the network of microprocessor based devices. In assisting with this upgrade I was responsible for the design of the protocol, while Council Engineers managed the hardware design and selection for the new system.