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ABSTRACT This research study was conducted to develop and validate an appropriate methodology for determining capacity and level of service at signalized (service) interchanges for possible inclusion in a fixture edition of the Highway Capacity Manual. The research was sponsored by the National Cooperative Highway Research Program(NCHRP) and programmed as NCHRP 3-47, FY '94 ureter the title of "Capacity Analysis of Interchange Ramp Tenninals." The research agencies conducting the work were the Texas Transportation Institute of Texas A&M University and the University of Nebraska-LincoIn. The study considered a wide variety of field characteristics Hat might affect saturation flow, ~ncluding~ningradius,traffic volume, large use and downstream queue spiliback. A large program of field studies was conducted at 12 interchanges located in 5 states. A sizeable data base of over 51,000 queued vehicles covering 3,800 signal cycles of operations was electronically recorded. These data were used in some cases to directly develop analytic models of a statistical nature, whereas in other cases, the field data were used to calibrate, test, and verify various analytic models having been developed from a more theoretical basis. As an example, the Prosser-Dunne mode! was enhanced and tested extensively using a NETSIM-based experimental process. Other prototype software(INTERCHANGE)was developed to illustrate how a large group of interchanges configurations might be systematically analyzed for capacity and level of service. The research found that mobility at signalized interchanges is dependent on many factors Hat influence capacity. The importance ofthe prevailing factors may change depending on He general volume level and degree of existing congestion. Traditional prevailing factors affecting capacity include the interchange geometry, traffic mix, and signal green splits. The elect of queue spilIback on upstream saturation flow was modeled and verified with field data. This mode! could prove very useful for providing reliable operational analysis of traffic conditions near capacity-flow levels. During periods of oversaturation where the arterial links are filled with queues, additional non- traditional factors come into play. Dunng oversaturation, the upstream input capacity becomes highly dependent on the downstream signal timing and capacity to keep the output link flowing. Upstream input flow cannot exceed the total downstream capacity, and may be even less if demand starvation occurs. Signal offset during oversaturation was found to be the most important factor in determining which upstream signal phases can use the downstream capacity. This offset is not the offset that provides optimal traffic progression during uncongested flow periods. This report wall be published as a NCHRP report at a later date. v
