Maryland, Missouri and Ohio with I-270 in the highest... That’s 3/4 the I-270 variants! Interesting seeing it 3 times on the list, as someone that lived in StL and Cbus in the past, and visited DC a couple of times.
For the mile markers, 14 for STL’s 270 makes sense, as a lot of drivers use that part of I-270 to change from I-64 (exit 12) to MO 364 (exit 16) or I-70 (exit 20).
33 for Columbus’s I-270 is different from my guess, which is 23 (the stretch between OH 315 and I-71), but I guess New Albany developed to a point that part of I-270 to change from OH 161 to I-670 is much more used, also the Easton Mall’s exit is 33. A project that added C/D lanes between I-670 and Easton also finished last year in that area.
Additional context for Ohio:
For most of 2019, 14 was the last exit on I-680. At mile marker 16, the only traffic remaining on I-680 are the ones coming from WB I-76 and going to EB I-76, so it was pretty much a glorified ramp at that point. A new exit 16 opened to OH 164 right before the merge with I-76, so that may change the AADT data a bit.
Mile marker 108 of US 33 is where its western interchange with I-270 is. A lot of residents in Dublin use that portion of US 33 to/from I-270 to one of the two Dublin exits on US 33, Avery-Muirfield Dr (107) and Post Rd (106). The Dublin traffic makes up about half of those on US 33 at that point, as the AADT drops to 40k west of exit 106.
US 52 at mile marker 127 is across the river from Quincy, KY (with no river crossing ofc), and the area within 5 miles from that point in both directions is pretty isolated, with very few outside road access. An intersection with OH 125 is 4 miles east of that point, at mile 131. OH 125 acts as a shortcut for US 52 between I-275 and that point, as it's a straighter route, compared to US 52 paralleling the Ohio River.
The freeway section of OH 315 is naturally busy, connecting downtown and Ohio State to northern suburbs like Powell and Dublin. It's in the backyard of Ohio State, which contributes to a good amount of traffic on OH 315. What I don't get is mile marker 4. It's between the Medical Center (3) and Lane Ave (4) exits, the two exits serving Ohio State. Guess it's the combination of people entering on the previous ramp and exiting off the following ramp?
OH 371 is one of the shortest state routes in the state, and it's pretty much a driveway to a rangers station for a nearby state park.