Often, a road trip is as much about the journey as it is, the destination. It seems as though the designers of our interstate highway system had both in mind. Our highways snake across the country, passing through cities and swerving to avoid natural obstacles. As a result, some are far more direct than others.

For this post, I’ve mapped the longest interstate highways in the continental USA alongside the geodesic between each highway’s endpoints. I then graphed the differences in distances, and the respective sinuosities. The sinuosity of a path is simply its length divided by the shortest distance between its endpoints.

From these data, we can see that north-south trending highways (odd numbered) tend to be less direct, though I-35 is a good exception to that rule. I-25 is both the shortest and most sinuous highway shown, bouncing east to avoid passing directly through the Rocky Mountains. And I-95, which isn’t quite continuous, can’t be blamed for straying from the direct route, given that most of it would need to be a bridge over the ocean.

Among the east-west highways shown, I-20 has the lowest sinuosity, while I-90, which dips south to avoid the Great Lakes and Canada, has the highest. Note that I-90 is the longest interstate highway in the USA, but its geodesic is actually shorter than that of I-80!

Data sources: https://www.census.gov/geo/maps-data/data/tiger-line.html (primary road shapefile)