The R.H. Thomson Expressway was a planned freeway which would have cut through Seattle alongside the I-5 freeway with the same gusto that NYC’s BQE tore through the middle of Brooklyn. In the 1970′s the voters rejected the measure after lawsuits by a coalition of neighborhoods forced the city government to put the idea up to a vote.
It failed but a few on-ramps remain from the project. They’re scheduled to be demolished in the next few years as part of the 520 bridge reconstruction.
Personally I would prefer to keep the on-ramps and turn them into a walkway over Union Bay (they’re currently fenced off). Not only do they provide a lovely view but they’re highly symbolic of Seattle’s inability to make up its damn mind when it comes to transportation.