The best visualizations are information dense while being deceptively simple. This example takes the US interstate system and distills it into its key components.
From the post:
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.