Appendix B
DEFINITION OF LEVEL OF SERVICE (LOS) Levels of Service (LOS) are qualitative measures describing operational conditions of highways. Six LOS are defined for each facility type and are given designations ranging from "A" (the best) to "F" (the worst). LOS indicates quality of flow measured by a scale of driver satisfaction. • Level of Service A represents free flow. Individual users are virtually unaffected by the presence of others in the traffic stream. Freedom to select desired speeds and to maneuver within the traffic stream is extremely high. The general level of comfort and convenience provided to drivers is excellent. • Level of Service B allows speeds at or near free-flow speeds, but the presence of other users in the traffic stream begins to be noticeable. Freedom to select desired speeds is relatively unaffected, but there is a slight decline in the freedom to maneuver within the traffic stream relative to LOS A. • Level of Service C speeds at or near free-flow speeds, but the freedom to maneuver is noticeably restricted (lane changes require careful attention on the part of drivers). The general level of comfort and convenience declines significantly at this level. Disruptions in the traffic stream, such as an incident (for example, vehicular accident or disablement), can result in significant queue formation and vehicular delay. In contrast, the effect of incidents at LOS A or LOS B are minimal, and cause only minor delay in the immediate vicinity of the event. • Level of Service D conditions where speeds begin to decline slightly with increasing flow. The freedom to maneuver becomes more restricted and drivers experience reductions in physical and psychological comfort. Incidents can generate lengthy queues because the higher density associated with this LOS provides little space to absorb disruption in the traffic flow. • Level of Service E represents operating conditions at or near the roadway’s capacity. Even minor disruptions to the traffic stream, such as vehicles entering from a ramp or 2 vehicles changing lanes, can cause delays as other vehicles give way to allow such maneuvers. In general, maneuverability is extremely limited and drivers experience considerable physical and psychological discomfort. • Level of Service F describes a breakdown in vehicular flow. Queues form quickly behind points in the roadway where the arrival flow rate temporarily exceeds the departure rate, as determined by the roadway’s capacity. Vehicles typically operate at low speeds in these conditions and are often required to come to a complete stop, usually in a cyclic fashion. The cyclic formation and dissipation of queues is a key characterization of LOS F.
Roadway LOS Report – Rezoning/Concurrency Reviews
B-1
September 2008
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