We have more than 4,100,000 ~~million~~ miles of highway in the United States, but only 48,756 miles of Interstate highway. That doesn't sound like most places are just off of a random exit, and with one glance of the map, one can see vast swathes of land nowhere near an Interstate highway. However, the system does carry about 1/4 of all highway miles in the country, so that's a lot of lightly-traveled non-Interstate pavement. Furthermore, revenues from highway users does not cover the cost of the Interstate system. The Highway Trust Fund has been shrinking, because the $0.184 per gallon tax hasn't changed since 1993, and the fund is projected to be depleted by 2028. The Federal government has shored it up multiple times with transfers from the general fund. Wisconsin has done the same, I know, and likely quite a few other states that I'm not familiar with, as well. In short, the massive subsidy to automobile travel, especially in rural areas, is not practical, because it is not sustainable.