That’s the interstate that goes up through Watertown, NY and up to the bridge at Ivy Lea to cross into the Great White North, lol.
I went through that area on a road trip in 2007. I followed the four-lane expressway south from Ontario 401, then over one two-lane suspension bridge to the customs area (the road remained two lanes through there), then re-entered the USA, where four-lane I-81 narrowed back to two lanes after a few miles to cross the other bridge.