A computer system was developed to integrate three pre-existing traffic control systems. These three systems were the traffic signal systems on SR-99 and SR-522 in northern King County and the freeway ramp metering system (FLOW system) on Interstate 5 north of the Seattle central business district. This project continued previous WSDOT research, described in the report "Arterial Control and Integration, Final Report," 1990.
The integration system developed in this effort consisted of a single microcomputer that communicated with both the mini-computer that operated the FLOW system and the microcomputer through which an operator controlled both arterial signal networks. To minimize development efforts and costs and to demonstrate the potential for adding integration capabilities to traffic control systems, the integration system relied extensively on the control systems' existing capabilities.
Tests of the integration system produced mixed results. The basic system design was flexible and met the needs described in the earlier WSDOT report. The control system also showed that it can use the data collected by one control system to adjust the control strategy of another, independent system. Unfortunately, the integration system was not a complete success and therefore was not implemented by WSDOT. The integration system suffered from unreliable inter-computer communications. The communications difficulties were caused by "off-the-shelf" computer networking software that was not sufficiently fault tolerant for real-time control system applications. That is, the integration system experienced intermittent communications failures between the control system computers. These failures disrupted system operation, and that disruption could have significantly degraded traffic operations. A system operator was able to easily fix the communications failures, but the integration computer could not automatically handle them.