Such is the scenario Australian designer Cameron Booth has dreamt up in two separate maps depicting both the U.S. Routes system and the U.S. Interstates as massive crisscrossing subway grids.
For a somewhat similarly themed speculative geography project, see the effort to transform the entire globe into the New York City street grid. To witness their hugeness, click on the magnifying glass at the lower-right of each image and open the image in a new tab.
[Via Laughing Squid]