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