A second side job of theirs is to blow their whistle and pull over motorcycles and exact a few riel from them for whatever reason they can think of. Their most profitable activity is pulling over trucks and trailers and exacting bribes to pass through. Although this is technically illegal it is common practice. There are some general guidelines for Phnom Penh traffic. Generally, the larger vehicles get the right of way.
Bicycles are on the far right, then cyclos, then motorcycles.
But this general pattern can be violated at any time. I've tried to work my way through traffic creeping along a bicycle who is slowly plodding along in the middle of the lane doing my best to control my impatience. Generally, traffic flows in one direction along a lane similar to the United States. However, it's common for motorcycles, cyclos, bicycles, trailers and cars to come at you in your lane or even to the far right of you, or directly on. Actually, vehicles can come at your from literally any direction - straight on, side ways, or diagonally.