The Bay Freeway, also referred to as the Mercer Street Connection, was a proposed elevated freeway in the South Lake Union neighborhood of Seattle, Washington, replacing 0.7 miles (1.1 km) of Mercer Street between Interstate 5 (I-5) and Aurora Avenue North at the Seattle Center. Planning for the freeway began in 1954, with the proposal for a freeway from Elliott Bay to the Central Freeway, later I-5, via Broad and Mercer streets added to the city's comprehensive plan in 1957. Funded by a bond measure passed by Seattle voters in 1960, plans for the newly christened and elevated Bay Freeway to serve a multi-purpose stadium at the Seattle Center were opposed by citizens groups at public hearings in 1967, forcing the Seattle Engineering Department to consider other designs. After determining that a cut-and-cover tunnel would not be feasible, a second series of public hearings were held in 1970, leading to widespread controversy and a civil suit launched in opposition to the freeway. The lawsuit ended in November 1971, with King County Superior Court Judge Solie M. Ringold ruling that it was a major deviation from the voter-approved 1960 plan, forcing a referendum to continue on with the project. On February 8, 1972, the Bay Freeway project was rejected by a 10,000-vote margin in a municipal referendum, alongside the repeal of the R.H. Thomson Expressway, postponing congestion relief on Mercer Street until the Mercer Corridor Project in 2012.

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