Paris has a population density of 20000/km², and Manhattan of 28,872/km².
Population density is correlated with car density up to a certain point.
Usually up to the point where giving 50-100sqm of road to 1.1 people traveling in a car prevents more people from living in the area (because all there is is roads and parking), and more scalable infrastructure is built.
> the city of San Francisco has a density of 2,419/km²
Where are you getting that number? Wikipedia has 7,194/km².