Peace Arch

The Peace Arch has stood on the border of British Columbia and Washington State since 1921.

Besides being the site of a major border crossing for cars, it's also parkland, accessible as a provincial park from the Canadian side, and as a state park (with an entrance fee) from the US side. As there's no fencing, and visitors are welcome to wander across the border, it must be a tricky for border guards to keep track of who came from where. Park visitors are required to carry immigration documentation, unlike any other park in either country.