That irony, however sad, is worth confronting. Was physical safety the only point of our breathtaking expenditure of lives, money, and goodwill after 9/11, or was the point also to defend our way of life? Have we remembered — in Barrett’s words — “what we are fighting for . . . as well as what we are fighting against?” Are we proud of becoming a country where we must show ID to buy a bus ticket — even when (as recently happened to me) you don’t buy the ticket until you’ve reached your destination? Ours is a government that has banned scissors from Liberty (as in, Statue of) Island — and we are, it seems, a people who don’t really mind.

Then again, maybe we’re beginning to mind a bit. Some memorial visitors aren’t entirely happy with its resemblance to a Demilitarized Zone; others aren’t quite ready to accept that police there might delete pictures from your camera. You don’t need to be a card-carrying member of the National Rifle Association to sympathize with the Tennessee nurse who faced possible felony charges for asking where she might check the gun she’d inadvertently brought to the memorial. Nationally, too, there are flickers of a renewed debate over how Americans balance security and freedom — whether it’s Tea Partiers protesting intrusive airport pat-downs or the New York Times’ series about whether it’s time for the pendulum to swing back toward “civil liberties and individual privacy.”

The Sept. 11 memorial’s designers hoped the plaza would be “a living part” of the city — integrated into its fabric and usable “on a daily basis.” I thought that sounded nice, so I asked Schneier one last question. Let’s say we dismantled all the security and let the Sept. 11 memorial be a memorial like any other: a place where citizens and travelers could visit spontaneously, on their own contemplative terms, day or night, subject only to capacity limits until the site is complete. What single measure would most guarantee their safety? I was thinking about cameras and a high-tech control center, “flower pot”-style vehicle barriers, maybe even snipers poised on nearby roofs. Schneier’s answer? Seat belts. On the drive to New York, or in your taxi downtown, buckle up, he warned. It’s dangerous out there.

Krystal Bonner contributed to this report.