Happens rarely around here on the city streets, but occasionally I will encounter one or two. They usually are moving against traffic and move over upon seeing me. If they are traveling in the same direction I give them room if traffic permits me to move over, otherwise I will call out, ring my bell, etc., which usually precipitates them moving aside. If they don't, I just slow and follow and pass when I can. This is accompanied usually by some gentle but suggestive comments.

Sometimes I come up behind a runner who seems to be on a 5 minute mile pace. Then I draft.