The Ambassador Bridge is one of two ways to cross between the US and Canada at Detroit & Windsor. There's also a tunnel. The bridge gets most of the industrial/commercial traffic, while most ordinary folks use the tunnel, although it also depends on what part of Detroit or Windsor you need to land in. The bridge and tunnel cross the Detroit River, which isn't really a river - it's a strait (which is what the French word détroit means) running in the Great Lakes system, between Lake Erie (which is a Great Lake) and Lake St. Clair (which is not, but it is big).
Detroit-Windsor is, last I heard, the busiest crossing between the US and Canada, and also, I believe, the only one where the US is north of Canada. Lots of commercial traffic - big trucks - but also lots of daily commuters going both ways. I've even heard first-hand accounts of people skiing over the bridge during blizzards to get to their vital job (e.g., hospital workers) on the other side!
And, by the way, the guy who owns this bridge (yes, a guy owns it - rather, his company does) is the same guy who owns Detroit's most famous ruin, the Michigan Central Train Station (MCS), an 18-story Beaux-Arts building that's been vacant since 1988 but still stands. You know the one. It's been in national and international media quite a bit in the past couple of years.