Advanced Traffic Signal Control for Diamond Interchanges

Most modern traffic signal controllers contain "advanced" features that are not often used but may improve traffic operations under certain conditions. The Texas Transportation Institute recently completed a research project for the Texas Department of Transportation, investigating how diamond interchange operations can be improved by using advanced controller features. The research evaluated advanced features available in the traffic signal controllers currently used for diamond interchange control in Texas. Eight potentially useful controller features were identified. The effectiveness of these features was evaluated with hardware-in-the-loop traffic simulation. The researchers considered the applicability of these features under different geometric and demand conditions and investigated the effect of detector technology and human factors issues on implementation. One of the main findings of the research was the potential usefulness of the separate intersection diamond control mode. The separate intersection diamond control mode is not commonly used, but, if applied judiciously under specific geometric and demand conditions, it can provide more efficient control than the three-phase or four-phase sequences typically used in Texas. The free separate intersection mode can significantly reduce stops at interchanges under low-volume conditions, especially if permissive interior left turns are allowed and steps are taken to reduce activation of the interior left-turn phases. The coordinated separate intersection mode has the potential to provide more efficient operation than the three-phase or four-phase sequence under certain conditions that can be determined with signal optimization software such as PASSER III, Synchro, and TRANSYT-7F.