Yes, but the Main St. in San Francisco hasn't been around nearly as long as SF has. When the downtown grids were laid out, what's now Main St. was underwater. I

Yes, but the Main St. in San Francisco hasn't been around nearly as long as SF has. When the downtown grids were laid out, what's now Main St. was underwater. It didn't exist as land, let alone a road that was either significant or central. By the time it was filled in, it was home to Charles Main's mining provision shop. The other way you can tell that it's not "main" is that it's parallel to 1st Street. Most cities don't have both a Main and a 1st, they have one or the other (which is why "Second Street" is the second-most common street name in the United States behind "Main", with "First" coming in third place). If SF were using Main in the "normal" sense, it would be where 1st is, next to 2nd. Failing that, it would at least be the next block over from 1st, so that 1st was counting from it. Washington, DC, sort of works that way, where there's a "zero street" (North/South Capitol), and then 1st is the next block over. But... SF's Main St. is a full three blocks from 1st. It's just kind of in the middle of nowhere, logically speaking. And that's again because of the landfill situation - 1st St. is the first full block counting from the shoreline at the time the streets were built.