The obvious problem with these ferries is, what if you are on the other side of the river and there is no one on the otherside that needs to come over. The solution would be to make alternating ferries, one slightly up stream and setup the cable system so that when one ferry starts to move, the other ferry moves at the same time. Or, you have a ferry operator with a signal system but then someone has to be on the ferry 24/7.