Impacts of Replacing a Tight Diamond Interchange with a SPUI

Book excerpt: Tight diamond interchanges (TDI) and single point urban interchanges (SPUIs) are generally types of interchanges that can be used interchangeably in areas where right-of-way is constrained. There are limited amount of research conducted regarding the operational analysis and comparison of these two interchanges. The research that has been done regarding this topic are outdated and generally conclude that SPUIs perform better or about the same. Without knowing the true performances of the two interchange designs, it would make it difficult for engineers and planners to choose between the two interchanges. This research study will use the current updated version of the micro-simulation software, PTV VISSIM to analyze and compare the performance of the SPUI and the tight diamond interchange. The research will look at reconstructing a current tight diamond interchange into a SPUI in Wilsonville, Oregon to see how it will perform. The analysis was conducted for 7 varying volume levels for both the interchanges including the existing volume conditions. The volume conditions include multiplying the ramp volumes by 0.75, 1.25, and 1.4 as well as doing the same for the eastbound and westbound movements. The results of the analysis shows that the SPUI performs similarly compared to the tight diamond interchange for lower volume conditions, but perform very poorly for higher volume conditions. The SPUI in this location is more sensitive to volume change and does not perform as well as the tight diamond interchange. The SPUI performs better than the TDI by only 1.4 seconds for low ramp volume conditions with regards to average delay per vehicle and performs 31.2 seconds worst for high ramp volume conditions. Therefore, the SPUI should not be implemented at this location as it is not feasible for high volumes and not cost effective. SPUIs should be implemented carefully as it is not suitable for all places.