[ghaff]

Evacuating cities in the event of an atomic attack was also cited as a reason though that obviously seems questionable as well. As I wrote in another comment, the defense justification was probably mostly just a way to get the help get the act passed.

[bobthepanda]

Katrina and Harvey showed that at best, evacuating cities with highways is an extremely long, drawn out process that will most likely leave the most poor and destitute (who don't have cars) totally stranded.

[ghaff]

Yes. Basically you can't rapidly evacuate cities. Maybe highways help a bit but, if you can barely handle commuter traffic on a given workday, you're certainly not going to completely and quickly evacuate a city if there's some threat--and that's before you even get to the issue of people who don't own cars.

So the idea that highways would be useful if there were an imminent threat of nuclear attack looks pretty silly.

[bobthepanda]

It depends on how quickly and how far you need people to evacuate.

Lower Manhattan on 9/11 was evacuated mostly by foot and sea, with over half a million evacuated by boat alone: https://en.wikipedia.org/wiki/Maritime_response_following_Se...

[slededit]

They don't completely solve the problem, but it would be much harder to evacuate the cities without the highway system in place.

[bobthepanda]

Rail has upper capacity in the ranges of 60-90k passengers per hour, whereas car lanes are in the range of 2-3K passengers per hour in free flowing conditions. Granted that 60-90K figure doesn't involve people trying to shove furniture, pets etc. into their vehicles, but you could move quite a lot more with rail.

[slededit]

Rail is also used as part of the evacuation, but most people will want their car so they can carry enough supplies to last for the evacuation period. In fact one of the most egregious aspects of the Katrina disaster was New Orleans explicitly told Amtrak their trains were unnecessary [1].

>"We offered the city the opportunity to take evacuees out of harm's way," said Amtrak spokesman Cliff Black. "The city declined."

[1] http://www.washingtonpost.com/wp-dyn/content/article/2005/09...