In a CMBB scenario I'm currently working on, I have been able to get a handful of trucks to behave well by selecting the group and putting a single move line to a distant point. The section of the map has some significant woods and a bridge. I have adjusted the end point to land on the same road the trucks start on, with the result that the trucks move off in good order and find the road and bridge themselves on a line I would have selected for them. The result actually surprised me the first time it happened, I was checking reinforcements at the time and only needed them to move out of the way.
I think the key is that there is a large patch of woods near the reinforcement point that blocks the immediate movement line I drew. Note that halftracks are not as cooperative and seem to want to run in the grass at the edge of the road.
Diesel, I agree that scenario designers should consider what it would be like to manage the assigned reinforcements, and stagger them on successive turns if the immediate road congenstion is too great. Something I am careful with when building a scenario