SR was added to the state highway system effective April 1, 1992. SR 523 starts at Jct SR 99 at the north city limits of Seattle and heads east along N 145th St. As 145th crosses 1st Ave NE, it becomes NE 145th St. Crossing I-5 at Exit 175, SR 523 continues east on 145th to Jct SR 522/Lake City Way/Bothell Way and ends. SR 523 is 2.45 miles long.
At the time Seattle annexed north to 145th, the street centerline was used as the city limit line. Circa 1975, state law was changed to prohibit centerlines to be used as city limits. New cities and annexations to existing cities must use the right-of-way line down the side of a road rather than the centerline as the city limits. Existing city limits along centerlines were grandfathered into the law. Since Shoreline was incorporated and Lake Forest Park was expanded in the 1990s, the southern city limits of Shoreline and lake Forest Park run along the north right-of-way of 145th. Seattle's northern city limit is the centerline. Eastbound SR 523 and the southern half of SR 523 left turn lanes are in the City of Seattle but the westbound lanes and northern half of SR 523 left turn lanes are in unincorporated King County.
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