That's because the existing demand is highly under-satisfied, as evidenced by the existing traffic congestion. In theory you could actually satisfy all of it by building a thirty lane triple-decker highway and it would work. Of course, that's still stupid, because what you should do is build a normal highway (e.g. add a lane or two) to absorb some of the demand, and absorb more of it in other ways, as with mass transit or increasing zoning density so that people can live closer to their destination.