I think you overrate some of our city security luminaries.  They really don't
understand all that well.  What they do understand is that security is a hot
button issue and if they want their increased budgets and manpower and
prestige, they'd best be seen as protecting the citizenry which has even less
of a clue than anyone else.  Even in the cold war military, there was a
striking lack of appreciation of what a single trained and motivated individual
could do as far as collection of information and they were keyed to threats in
a way that few civilian planners are.

The scanning technology you speak of is not far off, but probably would not be
employed in the way you suggest simply because it would be so spectacularly
inconvenient to commuters that it would become a political no win for whoever
was doing it.  The trick will be to develop powerful enough scanners to screen
a wide area with multiple persons and then install it in such a way that people
are not constantly conscious of being scanned.

Wasn't it jjs who said...

It's here.  Rather, it's in New Carrollton, MD

One of the Metro (WashDC subway) stations has a sniffer installed
which puffs air at a passenger going through the turnstiles and
sniffs for chemicals.  

http://www.washingtonpost.com/wp-dyn/articles/A682-2004May4.html

"jjs" posted:
"... It shows no such thing. What it does show is how authorities and
politicians will test the public sense with outrageous assertions and
proposals so that they might exploit the public sense or nonsense to
implement social controls. ..."

True. It reminds me of an incident a couple of years ago here in Southern California. Some
was cruising the freeways, and using "something" to blow out car windows. Weelllllllll ...
some sub-moronic "CHP" officer got the hair-brained idea that "the perpetrator" was using
"Laser Pointers." Hysteria ensued among the Law Enforcement "types," and they were making
all kinds of "claims" and taking all kinds of "action."

At the time, Laser Pointers were also quite inexpensive. A cheap one could be bought for
less than $10, and they quite the rage with the late-teen and early-twentysomething crowd
... and a *lot* of kids were carrying them around "just for the fun of it."

Well, after several incidents where young people simply carrying a "Laser Pointer" were
stopped by the cops ... of virtually *all* the local departments (and there are roughly 90
different "law-enforcement" organizations in Los Angeles County alone) ... and treated to
what is termed a "Felony Stop of an Armed Fugitive (with a record of assaulting a Police
Officer)," someone else did a relatively scientific demonstration of just how ludicrous
this idea was. It also turns out that a small group of less-than-intelligent teens were
using a sling-shot and ball bearings ... because they liked watching the glass break, and
the people (in the car) freak-out!

BUT ... many people had been virtually assaulted by "The Police" ... with NO recourse for
being humiliated, roughed up, and sometimes thrown in jail - just for carrying a
completely non-violent toy.

Just another example of "how authorities and politicians will test the public sense with
outrageous assertions and proposals."