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Level of Service (LOS) is a measurement of the quality of traffic flow and congestion at intersections and roadways. LOS is defined by the amount of delay experienced by vehicles traveling through an intersection or on a roadway. LOS is based on an A-F scale with LOS A representing little or no delay and LOS F representing very long delays.

Under the Growth Management Act, each local jurisdiction is required to establish a minimum threshold of performance for its arterial roadways. Cities use this standard to identify specific actions to maintain the adopted LOS standard. The City of Mercer Island has established its Level of Service standard at intersections of two arterial streets as LOS C within and adjacent to the Town Center and LOS D elsewhere. This standard applies to the operation during either the AM or PM peak periods. The intersection of SE 53rd Place/Island Crest Way does not have sufficient volumes on SE 53rd Street to warrant a signal, and is exempt from the LOS D standard until traffic volumes increase and signal warrants are met.

To be consistent with the WSDOT standard for I-90 and its ramp intersections, the city will accept LOS D at those intersections. I-90 is designated as a Highway of Statewide Significance under RCW 47.06.140.