They started making arrangements to have a one-way only freeway early on but it takes time blocking off all the on-ramps and off-ramps to make it safe. Even with an all one-way freeway you are still going to have a huge backup with a bottleneck where you exit all that traffic, it's just that you won't see it in Houston. You have to provide the traffic traveling on the wrong side of the freeway a way to exit somewhere and not everybody wants to exit at the same place. People are ignoring the traffic cones and normal traffic delineation markers and markings which causes delays and hazards that the government can't control.

The reality is with all the freeways in a one-way direction they aren't large enough to handle the 4.5 million people of the greater Houston area that are fleeing. If you live in such a large populated area you are going to experience major delays.

Imagine a nuclear waste scenario where everybody in Houston needs to evacuate in one day.
There is nothing the federal government could do to ease the flow of evacuees on the roads.
The best thing I could think of is every C-5 transport aircraft available to evacuate people but that would still take for ever and the traffic at the airport would be horrendous not to mention parking.