If I were evacuating from Rita after seeing the evacuation from Katrina, I would have started by going South.
"South" out of Houston requires a boat.

Roads heading east were mostly open, though that was because everything in that direction was either destroyed or booked up due to Katrina. Also note, Rita ended up hitting east of Houston. My neighbors who evacuated to the east ended up in worse conditions than those who stayed here!
-Blast