On the US Interstate Highway System’s numbering plan, one- and two-digit interstates ending in an even number generally travel east/west, and those ending in an odd number generally travel north/south. The interstate numbers are lowest in the south and west, and get larger as they go north and east. Additionally, interstates ending in 0 or 5 are major cross-country routes.
Thus, if interstates followed the numbering system perfectly, the divisible-by-5 interstates should form a grid, from I-5 in the west to I-95 in the east, and I-10 in the south to I-90 in the north. I wanted to draw this grid to show where the major highways intersected.
However, a few of the divisible-by-5 interstates don’t quite conform to a grid. I-50 and I-60 don’t exist (since they would run through the same states containing US-50 and US-60, which could be confusing). I-85 crosses I-75 and ends up running west to I-65. I-30 and I-45 are both rather short interstates rather than the cross-country routes their numbers would indicate. And there are many instances where “parallel” interstates intersect.
Thus, the map got a bit more complicated than a simple grid. Inspired by Cameron Booth’s Interstate Highways as a Subway Map, I decided to use subway map language for my own project, while still keeping mine as close to an evenly-spaced grid as possible rather than a spatially-proportionate map. I also made the design decision to keep the intersections as metro areas rather than specific cities and suburbs for simplicity.