It took my sister about 33 hours to get from Galveston to Dallas on I-45, though that also includes her pulling off the road to take an hour-long nap a couple of times and her waiting in line for over half an hour to refuel. She said the first 30 miles (to one of the ring roads south of Houston) took about 45 minutes, and the last 170 miles to Dallas took about 3-1/2 hours ... it was the 100 miles in the middle (around Houston) that were horrendous. Oh, and she's pretty well "equipped to survive" because she's big on camping and climbing and such -- she had her tent, sleeping bag, thermarest, lantern, etc. along with her photos, CPU and other irreplaceable stuff. But even she ran out of water about halfway through that trek -- she started with about 7 gallons, but ended up putting about 2 into Wally (the dog, who has a heavy black coat and was suffering because it was 100 degrees and the a/c was off to conserve gas), 3 into the car (which kept overheating because ditto), and 2 into herself.

And to answer Arney's question, it looked to me as if the folks in and around Houston didn't bother waiting for the coastal residents to have a clear shot at getting inland, before stampeding onto the roads as well. Don't even get me started on the idiocy of the state/local authorities in delaying the implementation of contra-flow on I-45 and I-10 ... and then only starting it (at least on I-45) far north of Houston, leaving miles of empty highway on the south-bound side when the north-bound side was literally a parking lot.