Roads shut down. But haphazardly. Vast majority of cabbies refused to risk getting pulled over by the NYPD. Especially since most of them were the wrong ethnicity for that day. (Or could easily be mistaken for such.) Still, no one thought 9/11 was the Apocalypse. Cash still ruled the day. Both licensed cabbies and co-workers with cars were happily willing to chance it.... for a price.
One individual who years later became a co-worker loved to brag about how much he cleared that day simply driving co-workers home from a job in which most of them took public transportation. He wasn't well-liked, and quite frankly didn't care much for his co-workers. Apparently he made about three runs that day piling way too many co-workers into his large SUV. It was cash only, and no one rode for free. Needless to say, the ones who carried no cash were screwed!
He raved how he made just over $5K by making three trips to pick up more co-workers. And the ones who got a ride were still grateful despite his gouging. He did mention how one rather attractive Soccer mom co-worker who carried no cash offered him "something else" in exchange. But her husband had a very unsavory reputation. To put it mildly. So he decided not to take her up on the offer.
World doesn't need to come to an end for C.C.s and debit cards to become useless. During those times, people will happily accept cash since they know things will soon return to normal.