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This guest post is from Steve Revilak, Quartermaster of the Massachusetts Pirate Party. Interested in writing a guest post? Get in touch at Hive@Boston.com.

On July 4th, there were several Restore the Fourth demonstrations in Boston, and we Pirates were proud to participate. They were good demonstrations, and it was encouraging to see how much the issue of mass surveillance resonated with people.

After the day’s events, I came across this tweet from @AnonInMass, regarding TSA bag searches on the MBTA. We’ve all heard about the TSA’s bag searches (and we demonstrated against them in February). But @AnonInMass’s tweet got me thinking: how effective are these searches? In other words, what’s the probability that a TSA bag search will prevent a bomber from getting on to the subway?

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Let’s spend a little time trying to answer this question, even if it’s nothing more than a back-of-the-envelope estimate. This estimate involves assumptions, but I’ll try to do this in a way that casts the TSA in a positive light. In other words, let’s try to give the TSA the benefit of the doubt, and see how well they do.

First, we need to know something about the number of trips taken on the T. For this information, I’ll turn to the 13th edition of the MBTA’s Ridership and Service Statistics. Page 5 of the pdf shows the average weekday ridership of the red, orange, blue, and green lines is 719,934 (this is “Total Heavy Rail” plus “Total Green Line”). As far as I know, the TSA only conducts bag searches at subway stations; therefore we should only consider subway ridership.

Next, we need to know something about how ridership is distributed throughout the day; if the TSA were trying to maximize the effectiveness of bag searches, it would make more sense for them to search during peak hours, when more people are riding the T. I wasn’t able to find a breakdown of peak vs non-peak travel, so let’s say that 40% of ridership occurs during am peak, 40% occurs during pm peak, and 20% occurs during off peak.

Next, we need a notion of how long a peak travel period lasts. I’ll assume that peak travel periods are four hours long, based on the peak travel periods shown in commuter rail schedules.

Here’s what we have so far: 719,934 rides per weekday, where 80% (about 570,000) occur during eight hours of peak travel. From TSA’s perspective, this is the size of the problem space.

Now, let’s turn our attention to the screening process. I wasn’t able to find any information about how often bag searches occur, so for the sake of discussion, let’s assume that TSA conducts two four-hour screenings per day, each during peak travel periods. (Probably a high number, but again, we’re trying to give TSA the benefit of the doubt.)

The screening process involves wiping a rider’s backpack with a cloth, placing the cloth into a testing device, and reading back the test results. If the TSA can scan four bags per minute (one every 15 seconds), then they’re scanning 240 bags per hour. 240 scans/hour during two peak periods per day gives 240 * 4 * 2 = 1920 scans/day.

By combining this with the ridership figures we derived earlier, we get the probability that a TSA screening will catch a bomber:

(# scans)/(number passengers) = 1920/576,000 = 0.33%

Yes, all that for a 0.33% chance of success. Of course, that only holds if the would-be bomber submits to a scan (as opposed to, say, turning around and walking to the next T station). If you take the bomber’s motivation into account, then we can make an arguement that the odds are lower than 0.33%.

So far, we’ve been discussing what TSA subway screenings are capable of doing. We should also take stock of what the screenings cannot do. For example:

The TSA screenings won’t prevent someone from bringing a bomb onto a bus. Or a commuter rail train. Or a ferry boat.

The TSA screenings won’t prevent someone from bringing a bomb onto a green line street-level stop.

The TSA screenings may not prevent two people from bringing in pieces of a bomb and assembling them on the subway (e.g., separate components of a binary explosive).

The TSA screenings may not catch a bomber who’s wearing explosives on their chest (as opposed to carrying them in a backpack).

Fear can be a powerful motivator, but the point of this article is not to engender fear. Instead, the point is to look at the TSA’s subway screening program, to look at the problem it claims to address, and ask “does the solution fit the problem?”. Is it security, or security theater? Are the screenings intended to increase safety, or simply to condition us to being screened?