I'm not going to read all the posts, but in case it hasn't been mentioned:

In a large evacuation people it is only natural to move to the nearest safe place. This results in an overwhelming (to infrastructure and local authorities) crush of people. It also keeps those leaving later from being able to move into and through the area. If you want to avoid this you have to force the first to leave to travel further. If they can't get off the interstate they will keep going.