For some odd personal reason, and because I liked the writer, this article, about passing someone on a country path, made me smile.
http://www.theguardian.com/commentisfree/2016/apr/18/hello-country-walk-walkers-social--media-campaign-friendliness
As a generalization, in town or country, if there are few people out and about, and one walks by, I'll nod, maybe with a slight semi-smile, unless the person is carrying a dagger and frowning.. which luckily has never happened to me. Sort of an inbetween choice, as the person may be in no mood for a chat, or seeing a smiler.