I have a modest proposal
"If we were starting with a blank slate would we have the slow lane merging back into the traffic flow, or the fast lane?"
Simple. Remember Google is pioneering both driverless cars and SDN. So merging would be based on G-WRED. Low priority vehicles (ones that don't pay Google) would simply be randomly discarded. Google is currently working on the equivalent of the bit bucket, and ways to prevent motorists getting jittery. This may simply involve paying Google to become high priority, and thus reduce probability of the long drop.