They ran out of gas and/or their cars broke down AFTER they had traveled away from Houston. Columbus is 60 miles from Houston and cars were sitting in traffic jams for nearly two days running (some didn't think to turn them off while waiting) and ran low on gas.
Are the back roads clogged also? There is no way I would sit on a freeway for hours. That's what median strips are for.
Temps yesterday were in the high 90's - low 100's all along the routes. Overnight low temp is 80 degrees at the lowest. You can not sit in a crowded car or even an uncrowded one at midday without having the a/c running, and for that you need the engine on.
If your car is not in the best of shape this will cause overheating since the airflow for the radiator and engine cooling is provided by the movement of the car at hwy speeds.
Sort of a, d@mned if you do, d@mned if you don't, situation.