TX-36 is another option for evacuation, but it's only a single lane in each direction and only goes to small towns (Needville, West Columbia) so it should be just a secondary option. Going north on TX-288 then probably take TX-6 North around the city of Houston might be the best way to evacuate.
And for weaker hurricanes (cat 1-2) landing in the upper Texas coast we might only need to evacuate the coastal cities like Freeport and Galveston, the residents of Houston should be just fine to remain in the city.