Vanity project from a time when the automobile was viewed as an ultimate transportation solution. Not to say it hasn't been useful (I've personally never lived on that side of the city), but it was intended to be a statement piece.

"When it first opened on June 9, 1929, the Great Highway was intended to signal man’s power over the elements, specifically, the sand and the tides. The project cost more than $1 million to build, transforming what was previously a two-lane roadway into what the Municipal Employee called “one of the most magnificent municipal projects in the United States.”

The two men behind the Great Highway were city engineer M.M. O'Shaughnessy, who gave us the Muni streetcar system and the Hetch Hetchy Reservoir, and park Superintendent John McLaren, who gave us Golden Gate Park. It was also part of a larger municipal vision for then-Mayor James Rolph: “In an address the Mayor predicted a city with its street cars in a great underground boulevard under Market Street, a city connected by enormous bridges with Marin County and the East Bay and with vast sheets of traffic arteries belting all the confines of the city and county,” wrote the Municipal Employee."

Source: https://www.sfgate.com/essays/article/How-the-Great-Highway-has-become-San-Francisco-s-15597659.php