In a full-on nuclear exchange, it would be much more than major cities getting attacked. I live about 5 miles from Lincoln Labs and about 6 miles from Raytheon, so I'm screwed anyway. But, every ICBM installation, every port, and every military base would also be attacked in such a situation. I'd love to see some specific data on it, but I'd bet when you add those targets to the major cities, the vast, vast majority of the US population wouldn't fare too well.