What is the Base Rate of Terrorism?

This is dumb

Los Angeles Metro will be the first U.S. transit agency to use a security system created by the federal Transportation Security Administration to scan riders as they enter the system, the agency announced this week. New York and San Francisco has also been testing the technology, which TSA says will thwart terrorism or mass shootings. The agency says it plans to install the system at transit stations around the country, the New York Times reports.

Why is it dumb?

First, ut degrades the experience of passengers by making the environment more hostile, encouraging distrust, and slowing down people's movement. Security theater is dumb, and honestly I doubt it actually makes people feel safer. I suspect that if you act like there is a threat people will assume (not unreasonably!) that there actually is a threat. 

Second, its a misallocation of resources. The thing about security and "hardening" targets against terrorism, is that you can't defend everything because of resource constraints. 

To quote Frederick the Great: "He who defends everything defends nothing".Hardening targets against terror doesn't disrupt plots, it just causes them to shift their targets. In a city there are essentially infinitely many "soft" targets for terrorists.

Security measures like these (if they even work!) cost money but mostly just shift risk around, making places safer without making people safer. That's not always a dumb idea, but in this case it is - about 80 people die in transit rail accidents any given year, whereas there hasn't been a terror attack on any American metro in,  well, ever.

But the main reason that this is a dumb idea, the one that beats out all the rest, is this:

The base rate of terrorism is low

Terrorism is a tactic, it isn't defined by its weapons - terrorists don't have to use bombs.

Alternative definition

Terrorists can use guns, cars, knives, etc. And getting access to all tools of terror in America is frankly trivial.

I'm an Indiana resident writing this in Illinois. I could decide to commit an act of terror at 9, buy a gun in Indiana at noon, eat lunch, and commit an act of terrorism in Chicago before 5.  This would cost $1000, tops. Get me a pre-approved credit card and I'm set.

With even the smallest amount of planning  I could buy a gun on the internet and get it mailed to me, no questions asked. Or I could buy it from some guy at a gun show. Or I could 3-d print the parts and assemble it myself.

I could use a car or truck. If I don't own one I could rent one for less than $100 - or just steal, say, a UPS delivery truck.

I could use a blade,  or hell, I could just take a big stick to the metro and try to shove people in front of the cars.

But I'm not going to do any of those things - because I don't want to.

There's practically no barrier keeping a moderately-determined person committing terrorism. And yet terrorism is rare. Every day, hundreds of millions of Americans drive their cars, tens of millions have access to firearms, and essentially everyone could get access the tools of terror in a week (at most!).

Most days, most weeks, (hell, most months!) terrorist attacks in America do not happen.

Because most people don't want to commit acts of terror.

Maybe they're deterred by the threat of punishment, maybe they have some basic human decency, maybe they're just lazy. But they sure as shit aren't deciding not do it because of security checks at airports and metros.