Thanks a lot for this, it's a really cool map. Note that this is just the US numbered routes, so it's pre-Interstate.

I'm quite fascinated by the interstate system; I think it's one of the more amazing things this country's managed to accomplish in the last century. I admit that I draw my sample set from just my experience -- I grew up in the SF Bay Area, go to school near Chicago, and spend summers in Seattle, so all my travel is huge E-W distances -- but I think even if you're right, you have a good argument for N-S rail systems and there's still enough E-W travel that air travel is still quite necessary.