We really need a better roading algorithm. I built two parallel roads to speed up ware transportation, and all the traffic got stuck. Looking in detail, I notice there was a point where the two roads were connected, and the carriers were shifting all the traffic on just one road, possibly because it was one step shorter. One road unused, another stuck. somebody with the right skills should develop a sorting mechanism that allows for ring roads.
Was this a temporary thing?
I have done something similar, and a jam like you describe appeared for a short while.
I suspect that the router found a busy path and empty path, and started assigning all new stuff to the not busy new way. Since there were no carrier animals on the new road segmants, the road got badly stuck. When the new road stopped being significantly better, the situation stabilized.
I wrote this from memory, did not check code, could be wrong.