
The highest level "super highway" would have included pedestrian, rail, and cars.

Because all of these early plans were funded with local taxes, there was an emphasis on using transportation to work with land use in what we now call "transportation oriented development." Por ejemplo, shops and higher densities near the train stops. Then, came along the gas tax. It was seen as a way to pay for infrastructure, but there was push back against using it for anything other than automobile infrastructure (because they paid the tax).
The gas tax is mostly controlled by the states and federal governments. And at that level, they don't like to give away money without accountability and some "quality control." There-go, we have the beautiful uniform interstate highway system. And thanks to the downtown businesses desire for a connection to the suburbs, the interstate highways became inter and inner metro connection.

With all of the displacement, divided neighborhoods, and automobile traffic that came with the downtown freeways the push to remove them has grown and places like Portland have turned down proposed highways and even removed sections from the center. Transit-oriented development is the new buzz word and it's with good reason. Sometimes we were right the first time.
Read more in this article.