Of course, even if there is little risk to the site, the security measures may have some emotional value. The best argument I heard for enhanced security is the comfort that it gives some 9/11 family members and survivors. A representative for the September 11th Families’ Association told me they “appreciate all measures taken to insure the safety of visitors.” The president of the World Trade Center Survivors’ Network said much the same — though he also acknowledged a “diversity of opinion” among members. But not everyone is comforted by heavy security. John Mueller, a counterterror expert at Ohio State University and the Cato Institute, says that “visible security measures dealing with terrorism tend to make people more anxious about it.” He has research to back himself up.

If the purpose of elaborate security is to protect raw nerves rather than to address an active threat, it’s reasonable to ask how long it will be in place. Some memorial documents refer to controlled access during an “interim operating period” that runs through the end of 2013; others promise “open access . . . from all sides” only when the entire World Trade Center site (not just the main tower) is fully rebuilt. It’s unclear when that will be — 2016? TBD? (It wasn’t any clearer to a spokesman for the memorial.) And if the security measures are a response to an active threat, wouldn’t it be ill-advised to dispense with them the day the WTC site is finished?

For the sake of argument, let’s assume the memorial really is at risk. Are the current measures effective?

It’s not giving anything away to point out that the memorial’s security measures — at least the visible measures to which visitors are subjected — will not prevent a well-planned attack. Many of the most onerous requirements, even if they were effective deterrents, are unevenly enforced. For example, the requirement to provide names, addresses, et cetera in advance is undermined by the occasional distribution of a few same-day, timed tickets at several downtown locations and by the controversial ticket allocations to tour operators. (If an additional goal of named, prebooked tickets is to prevent reselling and control capacity, then the memorial could simply get rid of all tickets and count visitors as they enter and exit.) Then there’s the photo-ID rule, without which the requirement to give your name is meaningless. It’s at once overly restrictive — how many 13-year-olds have photo ID? — and largely unenforced (my ID was requested on only the first of my three visits).

I talked to Bruce Schneier, a leading thinker on security and the man who coined the term “security theater” to describe measures that are visible or intrusive but also pointless or ineffective. Schneier responded to a description of the memorial’s visible security with a pointed question: Is the memorial to the victims — or to our collective stupidity? The tactics, Schneier said, “assume we can guess the plot. But as long as the terrorists can avoid them by making a minor change in their tactics or target, they’re wastes of money.” What isn’t a waste of money? “Investigation, intelligence, and emergency response — stuff that doesn’t require you to guess the plot.”