Shouldn't this have been done already? After all, it is a mandatory evacuation. Why weren't the inbound lanes utilized sooner?
I have a map I keep in my car at all times that has all the county and farm roads in Texas. You could travel on 2 lane dirt roads faster than the interstate or go south and then to the west and then north. I've lived in Houston and the traffic is bad ALL THE TIME. I can't imagine how terrible it must be for these poor folks, especially in this heat.
Highways arene't train tracks...you can't "flip a switch" and make traffic go the opposite way.
The highway patrol/DPS people will have to clear the roads first, and stop people and/or put signs at every exit/on ramp.