If I'm riding with traffic on a moderately busy road and another cyclist is riding towards me, against traffic, how are we supposed to avoid a collision? Is there some unwritten or instinctive rule about which cyclist moves out of the way and in which direction?

I ride with traffic, as do most of the cyclists around here. But there are a number who ride against traffic. I encountered two of them yesterday with one close call. I've also had a few close calls over the years on a portion of a busy highway.