I read somewhere, some time ago, that the yellow lines were used in the center of roads where the traffic was moving in oposite directions and the white lines were used where the traffic in both lanes were going in the same direction. On a four lane hyway, the white were used to seperate the two lanes going one way and the yellow lines between the opposite direction traffic. Also the white line at the edge of the road was suppose to designate where the edge of the roadway was. Stay off the sholder, unless emergency. I know the yellow line in the middle comes in handy when it is so foggy that all you can see are the lines in the road. If you see a white line on the drivers side!!! LOOK OUT.
Sam