What US state capitals are not served by interstate highways?

Answer 1

The question was on a rerun of The Big Bang Theory tonight (it wasn't answered in the episode). Sheldon claimed there were four.

They are:

Dover, DE

Jefferson City, MO

Juneau, AK

Pierre, SD

Juneau clearly doesn't need one: Even leaving out that Alaska doesn't touch any other state making an "interstate" highway a bit of a misnomer (not that that stops Hawaii ... see below), there's nowhere even within Alaska you could drive to from Juneau. The city is surrounded by mountains and the ocean (the only practical way in or out is by air or boat). Any "interstate" in Juneau could at best take you to the other end of Juneau.

The other three are a bit more accessible.

Dover (population ~40,000) is not terribly far from an interstate (30 miles south of I-95 in Newark, DE), and connected to it by a divided, limited-access highway (Route 1).

Dover is on a peninsula, so any interstate to Dover would almost of necessity have to be a spur. It could theoretically link up with US-50 and its bridge across the Chesapeake Bay, but I can't tell if that's feasible given the terrain or not, and there aren't any major metropolitan areas along the way that you'd get as a bonus.

Jefferson City, like Dover, has a population of around 40,000. However, it's not connected to an interstate by even a limited access highway (US-63 from I-70 in Columbia, about 25 miles to the north, is divided, but is only limited access for a short distance, about halfway through Columbia itself). It's not like there's much else in the area, though; once past Columbia, you're basically driving through farm country until just across the river from Jeff City (when the road joins US-50 and becomes limited access again).

This one makes sense, because I-70 through Missouri exists primarily to link Kansas City on the west with St. Louis on the east. They COULD have routed it through Jefferson City, but that would have made it miss Columbia, which is significantly larger than Jefferson City and also where the University of Missouri is, so not really a net win. US-63 is probably adequate for the traffic Jefferson City gets. If a spur were to run from Columbia to Jefferson City and then another 60 or so miles south, it could connect with I-44 at Rolla (home of a major USGS branch and the Missouri Geological Survey as well as the Missouri University of Science & Technology). The problem with that is that US-63 is a "ridge runner" road, and it would be a major engineering feat to turn it into an interstate.

Pierre with a population under 15,000 is the second smallest state capital in the US, so it's not like it really needs an interstate highway. Still, I-90 does run fairly close (20 miles to the south), and US-83 (not limited-access, but it is at least a divided highway for most of the distance and it's in the middle of nowhere anyway) connects Pierre with ... the interstate. There's nothing at the junction of I-90 and US-83. Did I mention "middle of nowhere"? (There's something ... probably an unincorporated community ... called "Vivian" a short distance northeast of the intersection, but it's quite small... from the size I'd estimate a population of under 200.)

Pierre's case is actually the least explicable in terms of "oh, we skipped it so we could go through X instead"; it's not like there's anything else more important between Rapid City and Sioux Falls (in fact, if you're zoomed out enough on Google Maps to see both cities, the only things in between that even show up on the map are Pierre itself and Mitchell SD, population 15,000) ... it wouldn't have been much out of the way to have it pass through Pierre in between and still go through Mitchell, and wouldn't have missed any city of any significance as a result. I suspect someone with a say in the matter owned some otherwise worthless land they wanted to be able to sell to the government, though maybe I'm being unnecessarily cynical and there were terrain/geology issues that aren't apparent from the map.

Interestingly, Honolulu HI is served by an interstate (technically two: H1 and H201, and it only misses having three because H3 connects to H201 literally a few blocks outside the city limits).