I know this question has been asked of you before, but pray tell, if these people were on foot, where and how far were they going to go - and what were they going to do when they got there?
The problem is not that the bridge was "closed" by the local sheriff and they couldn't hoof it to Houston or even to Gretna. If they had gotten in, they likely would have taken to looting Gretna - certainly for water (the only thing essential at this time) and likely a lot else. That means breakins, fires, mayhem, possibly violence against whoemever was there. Chaos.
The problem was, aid was not allowed in by the State and the local government had completely broken down. Looting and vandalism was rampant in NO. This would likely have occured in Gretna, and the grannies trapped in wheelchairs at the Terrordome still would not have gotten food and water from those who had managed to make it into town.
Of course, Shep and Whorealdo would have been pleased...
I live on a dead end county public road with only a few handfuls of houses on it. No stores, no gas stations. If the same terrible situation happened here, rest assured we residents would close our road, a public road, from wandering people - some portion of which might be interested in a lot more than food and water. If we saw a real need and had the means we would provide help at the head of the road. But nobody is getting in unless they are legitimate law enforcement or accompanied by them.
Get a drink from a garden hose. Find some shade. Get away from the concentrated filth in the Superdome and convention center.
It wasn't like they needed to become full-time residents - just needed some kind of refuge for a couple of days. Is that so friggin' hard to understand?
The bridge was an OFFICIAL evacuation route. It wasn't a cul-de-sac. That's not even a valid comparison - the police force were preventing the lawful passage of people TRYING TO EVACUATE a flooded city.