Level of Service (LOS) is a quality measure describing operational conditions within a traffic stream, generally in terms of such service measures as speed and travel time, freedom to maneuver, traffic interruptions, and comfort and convenience.
The Higway Capacity Manual defines six LOS for each type of facility that has analysis procedures available. Letters designate each level, from A to F, with LOS A representing the best operating conditions and LOS F the worst. Each level of service represents a range of operating conditions and the driver's perception of those conditions. Safety is not included in the measures that establish service levels.
General Definitions of Levels of Service (LOS), more specific definitions are defined in the HCM.
- The Highway Capacity Manual (HCM) by the National Research Council
- AASHTO Geometric Design of Highways and Streets ("Green Book")
- Virginia Department of Transportation (VDOT) LOS Illustration