Being that I lived in NY, I can say with the utmost sincerity that the VAST majority of people walk the streets with their heads down, avoiding eye contact. So it certainly wouldn't surprise me if people didn't see anything... of course there are also the drivers on the street (who hopefully ARE using their eyes) and the people in the skyscrapers.

Although here's my story... I knew many people in the Towers on 9/11... and not one that I spoke to who were on the upper floors on the north side of Tower 1 saw the plane coming. I asked them all if they did. People in skyscrapers rarely look out the windows - it's "old hat" to them, and they're hopefully doing work, not daydreaming. :)

All my friends on the upper floors escaped, thankfully. Most were within 20 floors of the impact too. Amazing to me, to this day... They were damn lucky they were in Tower 1, because they knew to get out and had extra time since it collapsed second...