As you may realize from your own experience as a pedestrian, it really only becomes an issue when traffic becomes heavier and travels at a faster speed. Otherwise, roads are typically narrow—particularly outside cities—and drivers approaching each other navigate by some combination of trajectory, habit, and body language. When roads are so busy that it becomes a problem, some sort of custom or law takes hold. If you've traveled in much of the less-developed world, you know that "no harm, no foul" is the true prevailing rule in places like India or Vietnam.

As for choosing left vs. right, as I noted a month ago, "different types of transport, all used by right-handed people, tended to produce different rules of the road."