Hedberg Maps makes “A Numeric Topology of the United States Eisenhower Interstate Highway System “ which looks quite cool, though is not quite free. A full discussion is here
The interstate system has another quality besides the creation of corridors, boundaries and districts: it orders and grids the country. In creating the basic numbering plan for the highways, its creators followed a tradition that includes not only previous highway systems (including the 1920’s U.S. Highway System), but street layouts dating back to William Penn’s Philadelphia, the initial “nine squares” of New Haven, and the very definition of United States territory, the 1785 Land Ordnance with its grid of 6 x 6 mile townships. It has become so common for American cities to lay out streets in a square grid with numerical names that it can be surprising to go to countries where this practice is unknown. Learning to navigate even older American cities like Boston, where what grids there are are haphazard and streets change names seemingly at whim, can be daunting to those raised in orderly Omaha or Chicago.
Some ground-rules quickly emerged:
- I would try to keep the “5-roads” as my guideposts and conform everything else to them (but what do you do when 1-80 and 1-90 become one road in Ohio and Indiana?)
- One roadway = one line.
- Two-digit routes would be drawn with a heavier line weight than three-digit routes. Where they share a pathway, the heavier line takes priority.
- State boundaries would be topologically correct: every road intersection and state boundary road crossing would be shown in the correct order.
- Odd-prefix three-digit routes (i.e spurs like I-394) would be shown as straight lines, and even ones (i.e. loop roads like I-494/694) would be made of circular arcs.
- As much as graphically feasible, routes would be encouraged to lie along their numbered place in the grid for as much of their length as was graphically feasible.
- A minimum of 1⁄4 inch would fall between each major intersection. Mostly.
- I would use only straight line segments and arcs. No other curvy bits.