Most counties a US Route travels within one state

[getemngo]

Michigan's highest is US 31, with 14 counties (just barely entering Cheboygan before its northern terminus), followed by US 23 and US 131 at 13 each. I don't think any past route has gone higher. US 27 may have also been 14 if it entered Emmet County, which I don't think it did.

At various times, US-27 and US-31 both went into the UP, but only in Mackinac County.  And US-27 would have been in Emmet County on the approaches to the Mackinac Bridge.  That makes 15 for each of them.

I was counting Mackinac for US 27:

• Branch
• Calhoun
• Eaton
• Ingham
• Clinton
• Gratiot
• Isabella
• Clare
• Roscommon
• Crawford
• Otsego
• Cheboygan
• Emmet
• Mackinac

...but you're right about US 31. I forgot it once entered the UP. Oops!

[robbones]

US 67 in Arkansas traverses 14 or 15 counties

US 65 goes through 14

[Laura]

US 40 goes through 10/24 counties in Maryland (counting Baltimore City as a county): Garrett, Allegany, Washington, Frederick, Carroll, Howard, Baltimore, Baltimore City, Baltimore again, Harford, Cecil.

[okroads]

In Oklahoma, US 412 goes through 16 counties. US 62, US 64, and US 270 each go through 15.