"Least roaded" county

[SSOWorld]

Have you not forgotten Kalawao County?

[oscar]

^ No numbered highways, but the county's multiple roads are entirely state-maintained.

[MATraveler128]

I would have to say Piscataquis County, Maine fits the bill being that there aren't many numbered routes and it's probably more moose than people living up there. I don't count logging roads.

[gonealookin]

Nevada has some very large counties, and all of them contain at least one US Highway or Interstate within their borders, plus multiple state routes.

Nye County, generally northwest of Las Vegas, is the largest in the state at 18,181.92 square miles.  That's larger than 9 US states; for reference, if you combine Vermont and New Hampshire, they come to 18,965.52 square miles.  Nye has the fewest state-maintained highway miles per square mile in Nevada, at 31.68 square miles per highway mile.  Nevertheless Nye ranks #3 in the state in total state-maintained highway miles with 574.014 miles, including over 131 miles of US 6, over 108 miles of US 95 and over 312 miles of numbered Nevada state routes.

Lincoln County, north of Las Vegas along the Utah border, is the other one that has over 30 square miles per highway mile, at 30.08.  It's the third-largest Nevada county at 10.633.2 square miles (nearly identical to Massachusetts).  It has 353.459 state-maintained highway miles, including over 172 miles of US 93 and over 175 miles of numbered state routes.

[SSOWorld]

^ No numbered highways, but the county's multiple roads are entirely state-maintained.

Hawaii doesn't pay attention to numbers

[JayhawkCO]

Have you not forgotten Kalawao County?

It was my first thought, but the OP says contiguous US.

[Poiponen13]

Speaking of counties without Interstates and US Highways, is Harding County, NM a such?

[US 89]

Speaking of counties without Interstates and US Highways, is Harding County, NM a such?

Baaarely. US 54 comes very close, but it appears the boundary has been specifically designed to keep the highway and adjacent railroad entirely within Quay County.

[Poiponen13]

Speaking of counties without Interstates and US Highways, is Harding County, NM a such?

Baaarely. US 54 comes very close, but it appears the boundary has been specifically designed to keep the highway and adjacent railroad entirely within Quay County.

I thought that New Mexico doesn't have any non-federal highway counties.

[JayhawkCO]

Speaking of counties without Interstates and US Highways, is Harding County, NM a such?

Baaarely. US 54 comes very close, but it appears the boundary has been specifically designed to keep the highway and adjacent railroad entirely within Quay County.

I thought that New Mexico doesn't have any non-federal highway counties.

There are two: Harding and Los Alamos.