​These metal rings in the edge of the sidewalk.

One of the main streets in my town (rural Western Washington) has these rings set in the sidewalk where it meets the road. The street is nose in angled parking, so if you could see about a foot to the left, you would see the front bumper of my car. They’re spaced about 15 to 20 feet apart. Some have smaller rings. Nobody rides horses in town, so they aren’t for tying up a horse. They have to have a purpose or the city wouldn’t have installed them when it put in the sidewalk, and I have no idea how long that sidewalk has been there, but it’s in the oldest part of town, so it’s been there a very long time.

Any ideas?