OPERATIONAL CONSIDERATIONS FOR SYSTEMS OF INTERCHANGES

Basic three-dimensional considerations in design include the composition of the elements of the highway and its effects on driver operation from a dynamic viewpoint. This discussion is an extension of basic three-dimensional design and covers interchange operational requirements, route considerations, and related signing. The features discussed are not as direct and are perhaps much more subtle than those that have to do with the geometrics of longitudinal and cross-sectional elements. Design considerations under this heading deal mostly with communicative aspects between the driver and the freeway and interchange complex in which the main thrust is to clarify, simplify, and facilitate driver operations. There are 13 operational and design criteria associated with freeway and interchange design. They are basic lanes, lane balance, applications of auxiliary lanes, route continuity, appropriate interchange form, no weaving within interchange on freeway, right exits and entrances only, single exit on freeway per interchange, exit in advance of crossroad, simplified signing, implementation of decision sight distance, freeway and exit ramp speed relationships, and ramp spacing. Although these operational and design criteria are discussed in various chapters of the 1990 AASHTO Policy on Geometric Design of Highways and Streets, the focus here is to clarify their application in freeway and interchange planning and design. Many of the concepts were first developed in the late 1950s and early 1960s, yet most were not incorporated in the AASHTO design policy until 1984--some 25 years after inception. The paper is intended for use as a practitioner's checklist of the 13 essential criteria during planning and designing a new freeway facility or considering operational and design improvements to an existing facility.