Why are you looking at this old thing? Check out the newer, updated, better version here.
Elsewhere on the internet, there are other diagrams that are all highly simplified versions of America's Interstate system, and all are very interesting if not 100% accurate. However, despite being called "subway style" maps by many people who have found them, they actually bear little resemblance to the best examples of that genre. Typically, a good metro map (or diagram, to be more correct, as they are rarely geographically accurate enough to be a true map) uses colour coding of lines and a clear hierarchy of detail to present a lot of information very concisely and clearly.
So that's what I have set out to do. Drawing my cues from the original and best metro diagram, H.C. Beck's wonderful London Underground diagram, I have rendered the Interstate system in a much simpler form. I have made the "major" highways (those divisible by 5) the framework of the map, with the "minor" highways reduced in importance and rendered as thinner grey lines. Even with these highways, a difference in the greys indicates whether they are even-numbered (west-east) or odd-numbered (north-south). Dots on the highways indicate interchanges: large dots where major highways meet other major highways, smaller dots where major meets minor and tiny dots where minor highways begin or end. A full key at the bottom indicates clearly where each highway begins and ends.