Wikipedia nerds tell it like it is:
The [Seattle] geography of [Sir Mix-a-Lot's "Posse on Broadway"] is quite specific, going north up Rainier Valley on Rainier Avenue S., then east to "23rd and Jackson," a major intersection in Seattle's traditionally black Central District or "CD," then cruising east to Martin Luther King Jr. Way S. where "the set looks kinda dead" so "we need a new street," then by "23rd and Union" (still in the CD, but northeast), and then to Broadway. "The college" is Seattle University near the south end of the Broadway commercial strip, from which they drive to Taco Bell at the north end, which "was closed" (a local joke, as that particular restaurant has a never-lit and wholly inaccurate "Open 24 Hours" sign built into the wall) so they "go back the other way" to Dick's Drive-In, a local fast food institution and, according to the song, "the place where the cool hang out."