Originally Posted by Kestrel
Actually, I would submit that it doesn't change your throughput at all. Eventually, the south bound highway lanes need to switch back to the correct direction since there will be opposing traffic as you move farther north, and then guess what, you have to merge the people going north on the southbound lane back to the northbound lane, causing a jam. Ultimately, your throughput is exactly the same as having just the northbound lane. Also, if you jam up both sides, emergency services can't go into the city either.
Well, once the evacuation is over, you don't allow traffic on the southbound lane unless its heading southbound. As for emergency services, instead of having 4 lanes open, have 3 lanes open, leaving 1 lane for emergency services. Or 2 lanes in case emergency services needs to get by while another EMS is dealing with a traffic incident.
EDIT: And once the evacuation is over, and people head back, then you are fucked. Then only one lane can be open.