It seems intuitive that doubling the lanes doubles the throughput, but it doesn't because your bottleneck is farther north where everyone is going. Eventually, the converted southbound lane has to resume southbound service (esp as you approach cities like Dallas where people use the southbound lane) so, guess what, your northbound evacuation traffic has to merge back into the northbound lane. Which means taht there will be massive stoppage as eight lanes of highway try to merge back into 4.

So say you want to avoid that entirely and have people exit before a major metropolitan area (like Dallas). How many surface streets can handle several million cars? Probably not very many. Again, there will be major stoppages as several million cars try to exit on two or three lane exit ramps onto 3 or 4 lane roads.

Basically, all that doubling the lanes does is open up a parking lot on the other side of the highway.