Uniform Criteria for Level-of-Service Analysis of Freeways

For level-of-service analysis, the 1994 Highway Capacity Manual divides freeways into basic sections, ramps and ramp junctions, and weaving areas. Different criteria are applied to these components to define levels of service. This use of nonuniform criteria makes it difficult to assess operating consistency in terms of the spatial change of traffic operations throughout a freeway system. This problem can be alleviated by using vehicle speed to establish a set of uniform criteria for the systemwide evaluation of freeway design and operation. Toward this end, the relationships between speed and other related geometric design and traffic parameters should be established. The spatial variations of flow characteristics on freeways, and the need to account for such variations in capacity and level-of-service analysis, are discussed first by analyzing observed speed-flow relationships. This is followed by a discussion on the selection and application of uniform criteria for planning and design.