Cold planing is removal of the asphalt by not heating it. Usually a milling machine with blades that grind it away to the depth needed. Because the miller puts grooves in the road, you want that in the direction of traffic to prevent the bad ride if the grooves went across the road.
The machine picks up the millings as it goes down the road, and uses a conveyor belt to deposit them in the bed of a dump truck.
If you mill against traffic (going upstream) the dump has to cross oncoming traffic to get to the miller and to leave. Each truck has to completely stop traffic in both directions to do so.
If you mill with traffic, the dump just pulls out of traffic, then pulls back in, and only disrupts traffic moving in one direction.
"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928
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