Comparison of the Operational and Safety Characteristics of the Single Point Urban and Diamond Interchanges
Virginia Transportation Research Council, 1996 - Roads - 105 pages
The purpose of this study was to evaluate and compare the safety and operational characteristics of the single point urban interchange (SPUI) and the diamond interchange (DI) and develop guidelines that identify traffic and/or geometric conditions that favor one type of interchange over the other. State traffic engineers across the country were surveyed as to their experiences and opinions of the two types of interchanges in terms of operational performance and safety. Accident and operational data were also obtained for nine SPUIs and eight DIs. The interchange operations were studied using both field data and the computer simulation program TRAF-NETSIM. In addition, for each DI simulated, an SPUI counterpart was designed with the same geometric and traffic conditions for further comparison. No significant differences were found in the off-ramp, cross road, and overall interchange average delay. Ten volume scenarios were then developed and simulated for both interchanges at low- and high-volume conditions to analyze the effect of various traffic patterns on the relative operational performance of each interchange type. The accident data for the SPUIs and DIs were compared according to severity, collision type, and location on the interchange. No significant differences were found between the severity distribution and rates of the two interchange types. However, the proportion of on-ramp and off-ramp accidents was greater at the SPUI, and the proportion of accidents occurring in the center of the signalized intersection was greater at the DI. The proportion of angle accidents was greater at DIs than at SPUIs, whereas the proportions of rearend on-ramp, sideswipe, and fixed object accidents were greater at SPUIs. Vehicle conflicts at four interchanges in Virginia were also investigated and used along with the accident analysis results. The operational results, safety analyses, literature review, and survey of state engineers were used to develop guidelines to aid traffic engineers in the selection and design of the appropriate interchange type.
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AASHTO accident analysis accident data accident rates Arlington Boulevard average stopped delay cameras capacity clearance interval collision type comparison conﬁguration Conﬂict Data conﬂict study congestion cross road left-tum cycle length data collection delay per vehicle Diamond Interchange EB CROSS RD efﬁciency Federal Highway Administration ﬁrst frontage roads geometric indicated interchange design interchange forms interchange types left tums right left-tum movements left-tum volumes left-tuming vehicles major road marking lights NETSIM number of SPUIs off-ramp left tums on-ramp operational characteristics operational performance overpass Point Urban Interchange Questionnaire Survey ramp right-tuming vehicles roadway safety characteristics safety problems saturation ﬂow rates SB Off-ramp sec/veh sec/veh sec/veh sideswipe signalized intersection signiﬁcant difference simulation Single Point Urban Single-Point Urban Interchanges speciﬁc SPUI and DI SPUI design SPUIs in operation STOPPED VEHICLES total interchange trafﬁc conﬂicts trafﬁc engineers trafﬁc volumes type of interchange underpass Virginia volume scenarios WB CROSS WB CROSS RD