Koreatown in los angeles is a bit more than 1/3 the size of SF and achieves 42,000 per mile and is basically a mix of single family homes and 3-5 story apartments over parking. There is a subway that comes every 10 minutes. The 101 through here is not 15 lanes wide each, it's 4. The city still functions at this density, and could probably stand a lot more and function just as well.