I've driven around 271 several times. I'd say most highways are covered, it just comes down to new points becoming available, such as new construction, and people moving or getting new technology. It also comes down to your gathering equipment, too.
Here is a gif of my cleveland vs everyone map.
(Notice the concentration downtown)
I've basically done a little driving off highway and downtown coverage in places like Cleveland, Columbus, Cincinnati, Indianapolis... Louisville, Lexington, Richmond, Knoxville, Nashville, Memphis... uh... Shreveport, Little Rock.. Ft. Myers, Cape Coral.. San Francisco.... I also did a college tour in Michigan specifically for collecting. Those were pre- truck driving, when gas didn't cost $4+. I don't think gas was even $3. So I lucked out, big time, on when I became completely obsessed with this hobby.