Field Analysis of Operation and Design of Single-Point Urban Interchanges

Michigan is considering the much needed rehabilitation and upgrading of many of the freeway interchanges located in urban environments. The single-point urban interchange (SPUI) has been suggested as an alternative to designs presently used, in particular the Michigan urban diamond interchange. To evaluate the appropriateness of the SPUI, a literature review was conducted, along with an e-mail and telephone survey of other state departments of transportation experience with the SPUI. Then, a field review of selected SPUIs was conducted to collect observations about the geometric design, signal operation, pedestrian control, pavement markings, and land use and landscaping of SPUIs. A SPUI with the crossroad going over the freeway was found to be a preferred design. In addition, the smaller designs were observed to function the best. However, the use of continuous frontage roads is believed to counteract the advantages of the design. Currently, progression of the crossroad is not a problem, although the impact of the SPUI on intersections downstream must be considered. Additionally, the need for pavement markings in large SPUIs is paramount. However, these pavement markings overlap and cause driver confusion. Several other observations were also made: most SPUIs reviewed were not able to adequately provide for pedestrians, controlled access near the SPUI is suggested, and landscaping dramatically increases the aesthetics of the interchange. The SPUI is a good design and has a place. However, some of the newer and enhanced designs with the resulting increase in size may be counterproductive.