Supplies will never flow back.
If they have literally no other way to go and there is no different directions to choose from (or they both have the same % of blockage), they will just ignore traffic signs.
The Crossing near Bezet could just redirect all teh excess towards that asset.
But for all the other ones:
Where would supply capacity go, if not ahead?
Nothing about the traffic signs implies backflow. The truck points could simply stop as directed by the sign.
One or the other, the trucks should not ignore the traffic signs or the establishment of the traffic signs should be unavailable in the first place under those conditions.
The issue is not best course of action, the issue is that interface does not convey a consistent behavior to the player's command. Failing a command silently is bad design.
Nicht kleckern, sondern klotzen!
*Please remember all posts are made by a malevolent, autocratic despot whose rule is marked by unjust severity and arbitrary behavior. Your experiences may vary.