Believe it or not, that's what will happen. Some people will get in that traffic and realize they don't have enough gas, or can't get gas, aren't getting away fast enough, etc. and turn around and go home.
Knew a guy in Houma, LA that had that happen during Katrina.
Luckily, his property didn't get hit too hard, but he tried to evacuate and couldn't get anywhere so he went home.
TWC saying it's a CAT 4 now.