Every US combat division was designed to have and had enough trucks to move it's troops and it's supplies around within the area of it's operations. There were also supply formations like the Quartermaster Corps with trucks and drivers working in the rear areas. When the Red Ball Expess was created to overcome animmediate crises they merely created a new command. The units with trucks had to hand over all or a portion of them to the RBE. They also brought in new trucks and drivers.All it took was an order from some general. When the crises was over everybody went back to doing what they had been doing all along. Driving trucks. But, for their original units.