The ability of the public to use a highway facility to the capacity level for which the roadway was designed will depend to a great extent on the accommodations made for intersecting traffic patterns. The use of grade separations and interchanges can result in improved operations on both roadways. However, because of the investment required to provide grade separations, the selection of the highest priority locations and the design of the interchange are critical. One method of determining the priority of competing interchange locations is through a cost-benefit analysis. The Texas Transportation Institute developed a computer analysis program called Texas Ranking of Interchange Projects (TRIP) to accomplish a relative analysis of interchange cost reductions on a macroscopic level. The major economic benefits of an interchange include a reduction in delay costs, a reduction in vehicle operating costs, and the expected accident cost reductions. These benefits are summarized and matched against the expected construction and right-of-way costs of the proposed interchange. The selection of the interchange type is influenced by many external constraints. These constraints may include limited rightof-way, environmental considerations, historical structures, and handling traffic under construction. Interchange types include trumpets, diamonds, cloverleafs, and directionals. Each interchange type has specific advantages and disadvantages. The design selected must be constructible and maintainable under traffic and must be able to handle traffic movements into the ultimate design year. Experienced design, comprehensive research, and knowledgeable drivers are necessary to achieve the maximum highway user benefits from these facilities.

The ability of the public to use a highway facility to the ca-pacity level for which the roadway was designed will depend to a great extent on the accommodations made for intersecting traffic patterns. When these intersecting traffic patterns can be grade separated, the resulting roadway will function with increased efficiency, improved operations, and a higher de-gree of safety. However, it is not practical or cost-effective to grade separate all intersecting traffic movements. Since interchanges represent the most costly intersection treatment in terms of initial investment, the selection of the highest priority intersection locations and the design of the interchange are critical in providing an effective and efficient high-way system. An example of the order of the interchange se-lection and design process is shown in Figure 1. An interchange is defined by AASHTO as a system of interconnecting roadways in conjunction with one or more grade separations, providing for the movement of traffic between two or more roadways on different levels (1). Interchanges may be included in new location projects or in facility upgrade projects. Reconstruction work may improve an existing grade separation or provide a new interchange at the at-grade intersection of two or more roadways.