We are having fun in the G+ Boing Boing community. Our friend Rob Walker posted this photo and said:

I was in San Francisco recently and walking from Market/8th to the SF CalTrain station I kept noticing these odd little things, mounted about 10 feet off street level, a few-inches-wide tire-like object often (but not always) accompanied by small target symbols. What is it, is it functional, or some sort of street art thing. Anybody know?

What is this thing mounted on a San Francisco building?