Starting to go down a hill and the usual cargo truck is slowing down our lane. Our bus pulls out to pass the semi-trailer behind the cargo truck. All is well for about 3 seconds until the semi decides to pull out and pass. Now I am not sure about the unwritten rules of the road but it definitely looks like if you are only 1 oncoming vehicle, you have to yield. Every one can see this poor guy coming and they just keep passing. Now we have a loaded cargo truck on one shoulder, a semi-trailer in the middle and a chicken bus on the other shoulder with an oncoming car. Some how the semi does a braking maneuver that allows the car to just get by and leaves a cloud of smoke from the brakes while the chicken bus drops in behind the truck. The semi then finishes passing only to meet another oncoming pickup which is forced to the shoulder.