Smart Conveyor Modification

How hard would it be to have a smart conveyor that pushes the car that passes through/over it to the next slot with the least amount of wait time? I am constantly re-routing conveyors to move cars to slots that don’t fill as fast as others.

The cars always backfill on the conveyor slots when all the slots are full. But they back fill to the first available slot. It would be nice to backfill to the slot with the shortest time left before it finishes its cycle.