Image credit: Wikipedia/Brandonrush

A Small Peek at U.S. Interstate Highways

I was poking around recently on, noticed a recent update from the state of Iowa related to the National Bridge Inventory. I played around with it for a little while, but I didn’t have much time.

But it got me thinking about highway infrastructure more generally, and the Federal Highway Administration’s “Highway Statistics” provides some interesting data.

At first I looked at highway lane miles (a measure of highway capacity that is especially useful for budgeting maintenance costs) by state. The District of Columbia, when included among the states, has the fewest of all: 77. Then again, the district is only 68 square miles. The smallest state is Rhode Island, at 1,545 square miles, and is fourth on the list. Texas, it seems natural to say, leads the country in the number of Interstate highway lane miles.

Do states that are geographically large tend to have more Interstate extensive highway systems?

It turns out that lane mileage and geographic size have pretty low correlation (ρ≈0.24). On the other hand, the population and lane mileage are highly correlated (ρ≈0.91). This is not a particularly surprising result since there’s not much reason for a very large, but sparsely populated, place to have more than a few major highways.

The chart below shows the relationship between (Interstate highway) lane mileage and population, as well as the geographic size, for each state.

This peek — I’m loathe to call it an analysis — does not account for state and locally managed highways that provide additional capacity so it is certainly not a complete picture. It might also be interesting to look more closely at population density and regional variations. Alas, maybe next time.

Lane mileage is associated more closely with population than geographic size.

The R code for this chart is available on my GitHub repo.

Note: The chart above has been updated to correct the horizontal axis labels. They previously ranged from 0 to 16,000.