Level of Service

The concept of level of service (LOS) was introduced in the 1965 edition of the Highway Capacity Manual (HCM). It provided for the familiar letter-grade system for characterizing the quality of operations on a variety of traffic facilities from intersections to freeways. The LOS concept will remain primarily unchanged in the next edition of the HCM, expected in 2010. It will, however, introduce material directly related to user perceptions for the first time, and discussions surrounding LOS have raised interesting issues that may result in more extensive changes in the future. This paper attempts to address some of these issues in the context of the history of the LOS concept and its use in the planning, design, and analysis of traffic facilities. Among the major issues that should be thoroughly examined in the future is whether the concept is needed with the rapidly advancing state-of-the-art, which produces many quantifiable measures of service quality. The application of LOS to corridors, networks, and multimodal systems needs to be addressed, as it will differ from previous applications to points and uniform segments. Incorporating the results of research concerning user perceptions into the LOS framework has also raised interesting issues as the 2010 HCM has been developed. With the forthcoming 2010 HCM as a starting point, this paper explores current issues and makes suggestions as to how to address them while moving toward the as yet undefined edition that will follow the 2010 HCM.