An Operational Comparison of the Single-Point Urban and Tight-Diamond Interchanges

T he Florida Department of Transportation (FDOT) hired a consultant team to perform analysis of various grade separation concepts. This work was part of a project to determine the feasibility of using grade separation techniques to solve arterial/arterial intersection capacity deficiencies. A significant portion of this exercise was aimed at determining the relative traffic carrying capabilities of the tight-diamond interchange (TDI) and the single-point urban interchange (SPUI). This article presents a comparison using procedures developed for this project and the results. The initial phase of this project included a literature review to gather information on the design and operational capabilities of different grade separation concepts. The review indicated that several articles and papers have been written about the single-point urban interchange (often referred to as single-point interchange or urban interchange), and some of these articles made statements regarding the relative operational performance of the SPUI compared to the TDI. The TDI is illustrated in Figure 1 and the SPUI is illustrated in Figure 2. Some of these comparisons were predicated on certain traffic volume characteristics, while others were not. Some of these articles contained no supporting analyses for these statements, and others provided a partial description of analysis methodologies and assumptions. Perhaps most importantly, some of these articles presented conflicting statements regarding the performance of these two interchange types, and since none had complete documentation regarding methodologies and assumptions, the reader is left in a quandary as to what to believe. It is primarily for this reason that this article was written. The scope of the FDOT project ultimately would require the consultant team to assess the feasibility of constructing grade separations at seven sites, candidate locations based on future traffic projections, and identify a preferred alternative at each site. However, the scope also called for an initial “general” comparison of the grade separation alternatives. The general comparison and evaluation could then be used to quickly isolate the most appropriate alternatives for further study based on site-specific characteristics.