WSDOT sets level of service (LOS) standards for state highways and ferry routes of statewide significance (HSS) based on RCW 47.06.140(2). Regional transportation planning organizations (RTPOs) and WSDOT jointly develop and RTPOs establish LOS standards for regionally significant state highways and ferry routes (non-HSS) based on RCW 47.80.030(1)(c). LOS is based on peak-hour except where noted. Level of Service Standard is used by planners to determine the effectiveness of a roadway network. LOS assigns a rank (A - F) to road sections based on traffic flow and corresponding safe driving conditions. The Highway Capacity Manual and AASHTO Geometric Design of Highways and Streets ("Green Book") list the following levels of service: A = Free flow, B = Reasonably free flow, C = Stable flow, D = Approaching unstable flow, E = Unstable flow, E Mitigated = Congestion should be mitigated (such as transit) when p.m. peak hour LOS falls below LOS "E", F = Forced or breakdown flow This dataset is used as a general reference layer for geographic information systems at the Washington State Department of Transportation, and is updated as needed.