What is the longest business highway in the country? The first one to pop in my head is US 71B through Fayetteville, Springdale, Rogers, and Bentonville in Arkansas at 34 miles per Wikipedia (I'm not sure if this includes the orphaned stretch of the Fulbright Expressway (which should be I-149) or not.) BL 49 through Joplin and Carthage is pretty long too.
Texas seems to have a couple of 40 mile long beasts.
BL-85 from Lexington all the way to Greensboro in NC. It even has a concurrency with I-85 :sombrero:.
US 321 Business, through Dallas-Hickory, North Carolina, is over 37 miles long. I'm sure there might be longer in other places.
TX 75 is basically a business route, the old location of US 75 between Dallas and Houston that was bypassed when I-45 was built on mostly a new-terrain alignment. TxDOT inventories it at 132.6 miles. In terms of actual signed business routes, US 83 Business through McAllen, Harlingen, and other towns around there is the longest at about 46 miles, second place is US 287 Business through Fort Worth at about 39 miles, then I-20 Business through Midland/Odessa at about 32 miles, and I-40 Business through Amarillo at about 24 miles.
The longest in Michigan is BL I-69 through Lansing at around 14.7 miles. Previously, the Lodge Freeway (now, M-10) was BS I-696 at around 18.5 miles.