Smart people take the regular city streets all of the way out to back roads that lead out of town.
Dumb people take the main arteries and freeways.
I call it the Lemming syndrome. They just have to follow the ones in front of them no matter how slow it gets.
I watched this in hurricane Allen and was simply amazed that the evacuation from Corpus Christi was an eight to twelve hour ordeal when all they had to do is head west on a standard two lane highway, a few farm roads and "Viola!" they would be in San Antonio in about two hours.
People need to think for themselves...not let the government think for you.
KHOU mentioned highway 6, 35 and 71 out of Houston. They made the same point as you did - that often these roads are much less crowded. A current traffic report on those routes is coming up.
Having evacuated metro New Orleans twice now in less than 1 year I can say you're dead on correct. For Ivan and Katrina I left at about 3-4 am (24-36 hours before landfall) and took 2-lane highways to Houston arriving in roughly 7-9 hours. Others leave at 10am,head down the interstate and are shocked to find 80% of others with the same idea.