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[KCRoadFan]

Building upon the thread I created about the longest state highways that only went through one county, I thought of a similar question: when we break down interstate highways into segments based on the state, we find that there are some that only go through one county in that particular state. Among these single-county statewide interstate segments, which ones are the longest? Here are the ones I've thought of:

• I-40 in California - 155 miles, all in San Bernardino County
• I-15 in Nevada - 124 miles, all in Clark County
• I-90 in Pennsylvania - 46 miles, all in Erie County

Aside from the ones I've mentioned above, what other interstate segments are there like that, especially ones of comparable length? I'd be interested to find out more.

[jp the roadgeek]

I-81 in WV and MD: All 26 WV miles are in Berkeley County and all 12 MD miles are in Washington County.

You can make a case for I-83 in MD, since Baltimore City is technically not a county (36 miles)

I-95 in DE and NH: 23 miles  all in New Castle County and 16 miles all in Rockingham County

I-97 is all in Anne Arundel County, MD

I-84 eastbound in MA: all 7.6 miles in Worcester County (westbound lanes clip Hampden County)

[TheHighwayMan394]

MN's by default is I-394 at 9.7 miles all in Hennepin County. All other MN interstates enter multiple counties.

[Great Lakes Roads]

Indiana: I-469 in all of Allen County at 30.822 miles. Runner up would be I-865 in Boone County at 4.72 miles.
Illinois: I-294 in all of Cook County at 52.75 miles. Runner up would be I-270 in Madison County at 14.57 miles.

[GaryV]

Michigan has a few 3di's (I-475, I-675, I-194, unsigned I-296) but no 2di's.

[NWI_Irish96]

Indiana: I-469 in all of Allen County at 30.822 miles. Runner up would be I-865 in Boone County at 4.72 miles.
Illinois: I-294 in all of Cook County at 52.75 miles. Runner up would be I-270 in Madison County at 14.57 miles.

The end sign for I-294 is just north of Lake-Cook Road.

[DJDBVT]

I-93 in VT - 11 miles, Caledonia County

[CtrlAltDel]

Indiana: I-469 in all of Allen County at 30.822 miles. Runner up would be I-865 in Boone County at 4.72 miles.
Illinois: I-294 in all of Cook County at 52.75 miles. Runner up would be I-270 in Madison County at 14.57 miles.

The end sign for I-294 is just north of Lake-Cook Road.

We've discussed this before, and the consensus seemed to be that I-294 enters DuPage county near where those ramps to westbound I-88 branch off.

[SkyPesos]

I think the winners for both Missouri and Ohio are their respective I-270 variants:
Ohio: I-270, 55 miles in Franklin County
Missouri: I-270, 35 miles in St Louis County

[US 89]

Undefined in Utah as all five interstates enter multiple counties.

The same is true of New Mexico and Colorado (I-270 barely enters Denver limits). Curious if there are any others.

[webny99]

I-490 in NY is an interesting case. It's pretty close, but clips Genesee and Ontario counties on either end for 3 counties total. About 34 of its 37 miles are in Monroe County.

Other NY interstates that are entirely within one county...

-I-481, Onondaga County, 15.1 miles
-I-690, Onondaga County, 14.2 miles
-I-99, Steuben County, 13.1 miles
-I-290, Erie County, 10.2 miles
-I-787, Albany County, 9.6 miles
-I-990, Erie County, 6.4 miles
-I-590, Monroe County, 5.0 miles
-I-781, Jefferson County, 4.9 miles
-I-790, Oneida County, 2.3 miles

[SkyPesos]

I-490 in NY is an interesting case. It's pretty close, but clips Genesee and Ontario counties on either end for 3 counties total. About 34 of its 37 miles are in Monroe County

Similar case with I-465 in Indiana. Clips Boone County on the northwest corner for about 1 mile, and bumps into Hamilton County at its interchange with US 31 on the north loop for about 2.5 miles. Otherwise, it is in Marion County for the remaining 49 ish miles.

Other NY interstates that are entirely within one county...

-I-481, Onondaga County, 15.1 miles
-I-690, Onondaga County, 14.2 miles
-I-99, Steuben County, 13.1 miles
-I-290, Erie County, 10.2 miles
-I-787, Albany County, 9.6 miles
-I-990, Erie County, 6.4 miles
-I-590, Monroe County, 5.0 miles
-I-781, Jefferson County, 4.9 miles
-I-790, Oneida County, 2.3 miles

How about I-78 in Manhattan for less than a mile? 

[Dirt Roads]

All of I-485 around Charlotte are in Mecklenburg County, about 67 miles.  Amazingly, a little sliver of I-540 is not in Wake County, as the interchange with I-40 (starting point) is actually in Durham County.

[ran4sh]

In Georgia it is I-59 which is about 20 miles in Dade County.

The only other Georgia Interstates that go through just one county, are I-24 (also Dade County), I-520 (Richmond County), and I-516 (Chatham County).

[jp the roadgeek]

CT only has 2 that qualify.

I-291 at 6.4 miles in Hartford County.

I-684 at 1.41 miles in Fairfield County.

RI only has I-195 at 4.3 miles in Providence County

[SkyPesos]

Thought it would be interesting to pile up a leaderboard for this

(Also a correction to what I wrote above for Missouri: I-270 actually enters St Louis City for the last mile in the state, which disqualifies it from the thread. The new highest in the state is I-470 for 17 miles in Jackson County.)

State	Length (mi)	Interstate	County
California	155	40	San Bernardino
Nevada	124	15	Clark
North Carolina	68	485	Mecklenburg
Ohio	55	270	Franklin
Illinois	53	294	Cook
Pennsylvania	46	90	Erie
Indiana	31	469	Allen
West Virginia	26	81	Berkeley
Delaware	23	95	New Castle
Georgia	20	24	Dade
Maryland	18	97	Anne Arundel
Michigan	17	275	Genesee
Missouri	17	470	Jackson
New Hampshire	16	95	Rockingham
New York	15	481	Onondaga
Oregon	11	82	Umatilla
Vermont	11	93	Caledonia
Washington	11	205	Clark
Minnesota	10	394	Hennepin
Connecticut	6	291	Hartford
Colorado	0	-	-
New Mexico	0	-	-
Utah	0	-	-

[TJS23]

I just looked at Interstate 10 in Riverside County and it looks like it ties or beats Interstate 40 by a mile

[SkyPesos]

I just looked at Interstate 10 in Riverside County and it looks like it ties or beats Interstate 40 by a mile

I-10 also goes through LA and San Bernardino counties in the state.

[jlam]

I will contribute Kansas. Kansas's longest is I-235, which spends all of its 16.5 miles in Sedgwick county.

[Crown Victoria]

Looks like Florida's would be I-295 around and in Jacksonville, at 60.86 miles. I-295 does run along the border between Duval County and Clay County, but is entirely within the former. Florida also has I-595 (Broward), I-395 and I-195 (Miami-Dade), I-175 and I-375 (Pinellas), and I-110 (Escambia).

For South Carolina, I-185 is entirely within Greenville County for 17.7 miles. I-585 is also only in one county (Spartanburg), as is I-126 (Richland) and I-520 while in SC (Aiken). I-526 is longer and mostly in Charleston County, but briefly enters Berkeley County. I-385 was formerly only in Greenville County, but was extended into Laurens County during the 1980's.

For New Jersey, I-676 (3.84 miles, Camden County) beats I-76 (3.08 miles, also in Camden County) and I-278 (2 miles, Union County).

[TheHighwayMan394]

Iowa: I-880, Pottawatomie (16)
Montana: I-115, Silver Bow (1)
South Dakota: I-190, Pennington (1)
Wisconsin: I-894, Milwaukee (10)

North Dakota has no qualifying routes, as unsigned I-194 exists in two counties.

[MinecraftNinja]

New Jersey: I-76 in Camden County.

[empirestate]

-I-787, Albany County, 9.6 miles

Welllllll…

[US20IL64]

Indiana: I-469 in all of Allen County at 30.822 miles. Runner up would be I-865 in Boone County at 4.72 miles.
Illinois: I-294 in all of Cook County at 52.75 miles. Runner up would be I-270 in Madison County at 14.57 miles.

The end sign for I-294 is just north of Lake-Cook Road.

We've discussed this before, and the consensus seemed to be that I-294 enters DuPage county near where those ramps to westbound I-88 branch off.

294 is on the County Line, between 22nd and 31st street overpasses.
So, winner [for north 1/2 of IL] is I-190, in Cook, 3 miles. I-180 is in 2 cos.

[GaryV]

That's I-475 in Genesee Co., MI