Another exception is I-94 which runs from Sarnia, Michigan at the lower tip of Lake Huron to Billings, MT. Both go for quite a ways, but I-94 goes 1579 miles vs 887 for I-81. I expect there are more exceptions.I knew about the odd interstates being N_S and the even being E_W, but I didn't realize that there was a hierarchy of numbers with those ending in 0 or 5. And what about I-81, which runs from the Canadian border to Knoxville, Tennesee? I'd consider that one to be a "trunk-line" interstate.