The most impressive one I've seen personally is the north end of I-75 in Sault Ste Marie, Michigan. It ends at customs just as you get to the International Bridge that takes you into Canada, and that bridge is over the Soo Locks and the St. Mary's River, a key point in the St. Laurence Seaway taking shipping east and west from Lake Superior into Lake Huron and eventually out to the Atlantic Ocean. The sheer amount of commerce passing through that one area both east-west and north-south is staggering to think about.