There are also economic reasons. Parisian density is only possible with a great underground transportation system. That's hugely expensive to build and takes decades. SF has traditionally been a boomtown economy with corresponding bust cycles, so it's harder to make those kinds of investments. They are happening (e.g. the Chinatown extension) but more conservatively.

> Parisian density is only possible with a great underground transportation system. That's hugely expensive to build and takes decades.

It takes decades as an artifact of governmental processes. There's no necessary reason it should take decades.

Saigon where I'm back from is building a light rail to the center with twenty odd 30-40 story residential blocks along it. The whole thing is taking about 2 years to build.

That kind of thing is good for preserving historic buildings too - the blocks are all in nondescript areas 1-4 miles from the center.

Can you elaborate on Paris? It was my impression it was full of historical buildings and is expanding outwards, which isn't an option for SF.

It's an argument that gets made over and over again:

Paris: density 55,673/mi^2

SF: density 17,246/mi^2

(source: wikipedia)

To expand:

All of NYC - 28,053/mi^2 Manhattan alone - 72,033/mi^2 London - 13,410/mi^2

Bear in mind that's using the legal definition of Paris which excludes many of its suburbs.

The SF definition is also excluding its suburbs (i.e. Silicon Valley).

I'm mainly just talking about the density -- many more people fit into less space than in San Francisco. And its both an option for us to build up, and build in -- we could close down some of our wider car-oriented streets and build walking/biking oriented roads.