I've already told the story of being in Manhattan on 9/11. Through the day, vehicles could leave the city, but they could not come back (even taxis). That evening, most people who could leave Manhattan had done so. We were stuck until the next day. We ended up finding a restaurant for dinner in Times Square that was open.
Times Square was just totally bizarre. There were no cars at all. Perhaps one car would pass every five minutes. A couple of guys were roller skating around the streets, I supposed just because they could.
I was reminded of this experience by this photograph by Lucie and Simon, who take pictures of cities and digitally remove the cars and people.