It bridges Portage Bay on a viaduct, crosses through the Montlake neighborhood, and continues east on a causeway through the marshlands of the Washington Park Arboretum and across Foster Island. From there it crosses Lake Washington on the Evergreen Point Floating Bridge (1963) to Medina. At 7,578 feet (2,310m), it is the longest floating bridge in the world.
From Medina, it traces the border between Hunts Point and Yarrow Point to the north, and Clyde Hill to the south. Intersecting with Interstate 405 in Bellevue, it then runs into Redmond. It bisects the Microsoft campus, passes Nintendo of America headquarters and crosses the Sammamish River and Bear Creek, before ending at a junction with State Route 202. A set of ramps connecting SR-520 to Avondale Road NE were completed in 1996.
In the 1963 and 1967 revisions of the King County Streets and Highways Plan, SR-520 appears in its entirety. The segment from I-5 to I-405 is shown as existing or "to be improved"; the segment from I-405 to SR-202 is shown as "proposed".
The 1967 Puget Sound Regional Council of Governments recommended freeway system omits the segment from I-405 to SR-202, replacing it with a freeway from the Sand Point-Kirkland bridge (then proposed as a third Lake Washington crossing) along the current route of SR-908, deviating southward near West Lake Sammamish Parkway to end at the current SR-520 terminus. In this plan, SR-520 ends at I-405.
All planning maps from 1974 onward show the SR-520 routing as it currently exists.
When the new numbering system for Washington highways took effect in 1964, SR-520 ran from I-5 to the junction of Lake Washington Boulevard NE and Lincoln Avenue (now Bellevue Way).
By 1966, SR-520 extended east to I-405.
The segment from I-405 to 148th Avenue NE opened in the early 1970s. In the mid-1970s, the segment between West Lake Sammamish Parkway and SR-202 opened with the route number State Route 920 as a Super-2 freeway. This segment was widened to a divided 4-lane freeway by 1990.
The final segment of SR-520 between 148th Avenue NE and West Lake Sammamish Parkway opened circa 1979. At this time, SR-920 was redesignated as SR-520.
In the late 1980s, the shoulder of westbound SR-520 from Bellevue Way to the Evergreen Point Floating Bridge was converted for use as a HOV lane. In the 1990s, both sides of SR-520 from 124th Avenue Northeast to Lake Sammamish Parkway were widened to add a HOV lane, and collector-distributor lanes were added from NE 40th Street to W Lake Sammamish Parkway. A new interchange was built at NE 40th Street to accomodate expansion of the Microsoft and Nintendo of America corporate campuses.
The freeway revolt that stopped the R. H. Thompson Expressway had its origins in opposition to SR 520 itself. Architect Victor Steinbrueck, writing in 1962, objected to the "naked brutaility of unimaginative structures such as this proposed crossing of Portage Bay, which eliminates fifty houseboats while casting its shadow and noise across this tranquil boat haven.
|Seattle||0.00||– Portland, Vancouver||Westbound exit and eastbound entrance|
|0.19||Roanoke Street, Harvard Avenue||Westbound exit only|
|0.94||Montlake Boulevard (SR 513) – University of Washington|
|1.31||Lake Washington Boulevard||Westbound exit and eastbound entrance|
|3.07||Evergreen Point Floating Bridge over Lake Washington|
|Hunts Point||4.59||84th Avenue Northeast||Eastbound exit and westbound entrance|
|5.17||92nd Avenue Northeast||Westbound exit and eastbound entrance|
|Bellevue||5.97||Bellevue Way Northeast, Lake Washington Boulevard Northeast||Eastbound exit and westbound entrance; former SR 908|
|6.27||108th Avenue Northeast||No eastbound exit|
|7.52||124th Avenue Northeast||Eastbound exit and westbound entrance|
|9.17||148th Avenue Northeast|
|10.13||Northeast 40th Street||The Northeast 40th Street interchange was not part of the original SR-520 plan but was constructed circa 1998.|
|10.73||Northeast 51st Street||Completed in 1986.|
|11.79||West Lake Sammamish Parkway Northeast||Former SR 901|
|12.82||Avondale Road||Continuation beyond SR 202|