Level of Service (LOS)

Highway traffic congestion is expressed in terms of Levels of Service (LOS) as defined by the Highway Capacity Manual (HCM). LOS is a letter code ranging from A for excellent conditions to F for failure conditions. Complete free-flow operations with no restrictions caused by traffic conditions are described as LOS “A”. LOS “F” represents forced or breakdown of the traffic stream characterized by the familiar traffic jam. LOS “B” through LOS “E” describe progressively worse traffic conditions. The conditions defining the LOS for a highway are summarized from the HCM below:
LOS A • Level of Service A - Represents the best operating conditions and is considered free flow. Individual users are virtually unaffected by the presence of others in the traffic stream.
• Level of Service B - Represents reasonably free-flowing conditions but with some influence by others. LOS B
LOS C • Level of Service C - Represents a constrained constant flow below speed limits, with additional attention required by the drivers to maintain safe operations. Comfort and convenience levels of the driver decline noticeably. LOS C is the Colorado Department of Transportation's design service level (design capacity) for rural highways. (The majority of U.S. Highway 85 is a rural highway.)
• Level of Service D - Represents traffic operations approaching unstable flow with high passing demand and passing capacity near zero, characterized by drivers being severely restricted in maneuverability. LOS D is the Colorado Department of Transportation's design service level for urban highways. (I-25 and parts of U.S. Highway 85 are urban highways.) LOS D
LOS E • Level of Service E - Represents unstable flow near capacity. LOS E often changes to LOS F very quickly because of disturbances (road conditions, accidents, etc.) in traffic flow.
• Level of Service F - Represents the worst conditions with heavily congested flow and traffic demand exceeding capacity, characterized by stop-and-go waves, poor travel time, low comfort and convenience, and increased accident exposure. LOS F
(LOS information has been reproduced here by permission of the Transportation Research Board . Special Report 209: Highway Capacity Manual, Third Edition, Copyright 1998 by the Transportation Research Board, National Research Council, Washington, D.C.)


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