Number of counties served by interstates

[SD Mapman]

Nebraska:  16/93, or 17.2%.

that's what happens when the main north-south is on the other side of the river.

I'll bet North Dakota and South Dakota have a higher proportion.

You'd be correct, by my count:

North Dakota: 14/53, 26.4%

South Dakota: 21/66, 31.8%

Of course Nebraska's counties are also tiny compared to North Dakota's (which is why two main Interstates in ND run through less counties than NE with one).
Edit: Add missing comma because my OCD was going off.

Speaking of county size, Wyoming's percentage is 12/23 (52%). This is with THREE interstates. Compared with the East River counties in SD and ND, Wyoming's are huge.
(But I digress).

[Molandfreak]

This is with THREE interstates.

Wyoming has FOUR interstates.

[NE2]

This is with THREE interstates.

Wyoming has FOUR interstates.

[DTComposer]

I-605: Los Angeles (1)

The terminus of I-605 (I-405/CA-22 interchange) and its first exit (Katella Avenue/Willow Street) are in Orange County, FWIW.

[bassoon1986]

Texas has interstates running through 92 of 254 counties for a percentage of 36%. I only counted for I-69 the portions that are official. That's a pretty good percentage for such a huge state.

[roadfro]

So here's a question: what state has the highest percentage of its counties served by an interstate?

Connecticut has 7/8, or 87.5%. Can any other state beat that?
New Jersey has 18/21, or 85.7%. Close but no cigar.

And the obvious antithesis: what state has the lowest percentage of its counties served by an interstate? I'm guessing it'd have to be Hawaii, with 1/5, or 20.0%. Delaware has 1/3, or 33.3%, which is also pretty low.

Nevada has 17 counties, and a good chunk of them have an Interstate presence...more than I thought actually.

Nevada counties served by Interstates: 11/17 = 64.7%

• Carson City (I-580), Churchill (I-80), Clark (I-15, I-215, I-515), Elko (I-80), Eureka (I-80), Humboldt (I-80), Lander (I-80), Lyon (I-80), Pershing (I-80), Storey (I-80...barely), Washoe (I-80).
  Nevada's counties without any stretch of Interstate: 6/17 = 35.3%
  
• Douglas, Esmeralda, Lincoln, Mineral, Nye, White Pine
  
  Prior to the extension of I-580 to Carson City in 2012, the lists were 10/17 = 58.8% and 7/17 = 41.2%

[brownpelican]

MISSISSIPPI

I-10
Hancock, Harrison, Jackson

I-20
Warren, Hinds, Rankin, Scott, Newton, Lauderdale

I-55
Pike, Lincoln, Copiah, Hinds, Rankin, Madison, Yazoo, Holmes, Carroll, Montgomery, Grenada, Yalobusha, Panola, Tate, DeSoto

I-59
Pearl River, Lamar, Forrest, Jones, Jasper, Clarke, Lauderdale

I-110
Harrison

I-220
Hinds, Madison

[golden eagle]

Six more Mississippi counties will be added when I-22 is officially in business, putting the state's total to 34, out of 82.

[dfilpus]

North Carolina is in transition: There are Future Interstates and Interstate extensions being built in counties currently without Interstates.

Counties w/Interstates: 43/100.
Counties getting interstates: 3/100. I 73: Rockingham. I 74: Scotland. I-140: Brunswick.
Counties without Interstates: 54/100.

[NWI_Irish96]

In Indiana, I know that I-74 barely clips the corner of Rush County but does not have an interchange there.
I-65 passes through a corner of Newton County without an interchange.

I can't think of another county in Indiana that has an interstate pass through it without an interchange.

[JustDrive]

I-80 in California nicks the southeast corner of Sierra County without any interchanges. 

[SD Mapman]

This is with THREE interstates.

Wyoming has FOUR interstates.

Does it really count, though?

[hotdogPi]

I believe that Massachusetts is served by an Interstate in every non-island county.

[dfilpus]

I believe that Massachusetts is served by an Interstate in every non-island county.

Barnstable isn't really an island county.

[Rupertus]

Interesting. I'll do Michigan.

I-69: 8 (Branch, Calhoun, Eaton, Clinton, Shiawassee, Genesee, Lapeer, St. Clair)
I-75: 15 (Monroe, Wayne, Oakland, Genesee, Saginaw, Bay, Arenac, Ogemaw, Roscommon, Crawford, Otsego, Cheboygan, Emmet, Mackinac, Chippewa)
I-94: 9 (Berrien, Van Buren, Kalamazoo, Calhoun, Jackson, Washtenaw, Wayne, Macomb, St. Clair)
I-96: 10 (Muskegon, Ottawa, Kent, Ionia, Clinton, Eaton, Ingham, Livingston, Oakland, Wayne)
I-196: 5 (Berrien, Van Buren, Allegan, Ottawa, Kent)
I-194, I-275, I-296, I-375, I-475, I-496, I-675, I-696: no new counties

By my count that's 35 out of 83, for about 42%. Only 2 of those 35 are in the Upper Peninsula, which is almost 30% of the state's land area. None of these Interstates pass through a county without an exit, although I-75 comes within about 3/10 of a mile of clipping the corner of Gladwin County, near the Arenac-Ogemaw line (which I didn't know before this exercise). The shortest in a county is in my old home county, Emmet, which has only about 2 miles of I-75 before it heads up the Mackinac Bridge; however, there are 2 exits in those 2 miles, one of which (US 23) is somewhat important.