> I noticed this looking at a satellite photo of Nebraska recently — the towns lit up following a major East/West interstate across the state.

At least for the midwest - specifically west of Chicago, the way I always heard it was that the towns are fairly equally spaced (look at a night-time light map of the US) because there needed to be a rewatering / refueling place for steam engines, so towns popped up at the range of steam engine service intervals.

I bet the towns existed on a nice line (the rail line) first, and then it made sense to put the interstate parallel to the existing towns.