After almost 30 years of living in Michigan, I realized road names along particular county boundaries were the product of a naming convention. In searching for more occurrences of it, I found a couple more conventions, none of which are universal but which do seem to be partly regional.
In the heartland region, most prominently around the cities of Alma and Mt. Pleasant, roads along county boundaries have uninspired names like "County Line Road", "East County Line Road", and "Gratiot County Line Road". That's by far the most common convention, occurring here and there along the southern end of the state as well as in the northern half of the lower peninsula. It seems to be the only convention present in the upper peninsula, though the road segments with such names are comparatively shorter.
Around Detroit, boundary roads have hyphenated names: "Oakland-Washtenaw", "Washtenaw-Wayne", and "Monroe-Wayne". Further north but still on the east side of the state are "Lapeer-Macomb", "Bay-Arenac Road", and "Bay-Arenac Line Road".
And then the convention that started me on this deep dive: portmanteaus. One I used to drive past regularly is "Clintonia", a mashup of Clinton and Ionia's county names but which I always thought of as merely a variation on "Clinton". The pattern became apparent between Newaygo and Mecosta counties with its much more apparent "Newcosta Avenue". While there are a couple portmanteaus on the west side of the state (Kent + Ottawa = Kenowa, Ottawa + Allegan = Ottogan), the convention is most common around Osceola and Clare counties, north of the "County Line" fervor, where Mecosta + Osceola = Meceola, Lake + Osceola = Lakola, Clare + Osceola = Clareola and Clarola, Clare + Isabella = Clarabella, and Clare + Gladwin = Clarwin. (Honorable mention goes to the Musketawa Trail, a recreation trail that's not on the county line but which has one end in Muskegon county and the other in Ottawa county.)
Below is a color-coded map. Orange are "County Line" roads, red are hyphenated names, and blue are portmanteaus.
(This map isn't comprehensive. I put it together with a combination of Michigan county boundary data and Overpass query results, so depending on the precision of the boundary data, the vagaries of OpenStreetMap's data, and my scripting to assemble it all, some roads may have been missed. If you notice any, you're welcome to email me.)
I haven't checked other states to see if they have a similar tendency toward portmanteaus, but Michigan may have a predisposition for them. Out of 83 counties, about 20 have names of Native American origin, among them Ottawa, Mecosta, Newaygo, Osceola, and Washtenaw. Another 10 counties, including Allegan, have invented names meant to sound Native American. With such a supply of non-English (and made-up) names, it's maybe not surprising that new names should be conjured out of the mix and eventually accepted as names in their own right rather than derivations.