Risk-based screening does not treat all passengers as if each poses an equal threat. The goal is to allow screeners to concentrate more effort on passengers who present a higher risk while improving the efficiency of the screening system. This study concludes that such screening not only generally increases security and allows for more efficient and faster screening, but it dramatically reduces opportunity costs that deter travellers from flying, causes them to miss flights, or induces them to take a more dangerous means of transportation, e.g. the automobile, to get to their destination. Mark Stewart and John Mueller cast a critical eye on TSA PreCheck by evaluating the risks, costs, and benefits of risk-based screening.
Approximately $50 billion is spent annually world-wide in the quest to deter, disrupt, or protect against terrorist attacks to aviation. These are significant expenditures that have rarely been subject to systematic cost-benefit or risk analysis, and this leads to policies that are often excessively risk-averse and costly.
PreCheck seems to be one of the few programmes of the Transportation Security Administration in the United States that is risk-based – or at least it is one that screens passengers on the basis of risk. It does not treat all passengers as if each poses an equal threat. This allows for more efficient and faster screening, reducing opportunity costs that deter travellers from flying, causes them to miss flights, or induces them to take a more dangerous means of transportation (e.g. the automobile) to get to their destination. The goal is to allow screeners to concentrate more effort on passengers who present a higher risk.
We have systematically and transparently applied standard risk-analytic and cost-benefit methods to the issue of aviation security. We use the standard definition of risk where the ‘reduction in risk’ is the degree to which the security measure deters, disrupts, or protects against a terrorist attack. This model measures not only the overall risk reduction the security layers provide in total, but the risk reduction each layer contributes. The key issue is: does the security measure reduce the risk enough to justify its cost?
“…the layered system currently in place reduces the risk of such attacks by 98%…”
In our latest book Are We Safe Enough? Measuring and Assessing Aviation Security (Elsevier 2018) we have modelled the layers of security measures that have been put into place to deal with terrorist efforts to hijack an airliner and to down one with a passenger-borne bomb (see Figure 1). We found that, even giving the terrorists the benefit of doubt in many places in the analysis, the layered system currently in place reduces the risk of such attacks by 98% – and probably by quite a bit more. We also found this level of risk reduction to be very robust: security remains high even when the parameters that make it up are varied considerably.
“…we also find that the Federal Air Marshal Service (FAMS), at a cost of about $1 billion per year, fails to be cost-effective by a considerable margin…”
More specifically, we have applied our risk-based framework to evaluate the security measures designed to disrupt an effort to hijack an airliner. We find three of these to be highly cost-effective: crew and passenger resistance, the U.S. Federal Flight Deck Officer (FFDO) programme, and the potential addition of Installed Physical Secondary Barriers (IPSBs). However, we also find that the Federal Air Marshal Service (FAMS), at a cost of about $1 billion per year, fails to be cost-effective by a considerable margin – $1 of cost buys only 3 cents of benefit. A judicious shift of funds from FAMS to the other layers could potentially save both the taxpayers and the airlines hundreds of millions of dollars each at no reduction in safely1
The Risk Reduction Effects of
This approach can be applied to evaluate the Transportation Security Administration’s PreCheck programme, which seeks to make an expensive security layer – checkpoint screening – more efficient by directing airline passengers assessed as low risk to screening lanes that are faster and far less expensive to administer, allowing screeners to concentrate more effort on passengers who present a higher risk. Our analysis assumes that the terrorist boards in the United States, unlike the shoe and underpants bombers who boarded their US-bound aircraft abroad.
A large number of input variables are required to model the effectiveness of PreCheck. Many of these will only be known to TSA, so the following analysis is based on our best estimates. For our analysis, we make four assumptions:
- We assume that one out of every 100 million passengers is highly likely to be a terrorist who is boarding with an intent to hijack the airliner or to bring it down with a passenger-borne bomb. This is a very considerable exaggeration of the threat that terrorism presents to airliners under current conditions. After all, 839 million passengers passed through checkpoints in the United States in 2019. Yet, no passenger has sought to hijack an American airliner since 9/11, and none has tried to smuggle a bomb onto an airliner in the U.S. in more than 30 years.
- We further assume that 50% of all air travellers go through the PreCheck line.
- We also assume that TSA’s programme is 99% accurate in correctly identifying low risk passengers, but is less accurate (90%) in correctly identifying high risk passengers.
- Finally, we assume regular screening increases detection rates by 10-30% while PreCheck screening reduces deterrence and detection rates by 10-40%.
Our analysis concludes that the overall risk reduction from the full array of existing security measures is 99.3% for hijackings, and 98.0% for bomb attacks, i.e., a bomb attempt has perhaps one chance in 50 of being successful, and a hijacking attempt less than one chance in 150. When PreCheck is added to this array and when our four assumptions are applied, the risk reduction for the full array of security measures actually increases a bit. The total risk reduction remains at 99.3% for hijackings, and rises from 98.0% to close to 98.1% for passenger-borne bomb attacks — see Table 1.
|No PreCheck||With PreCheck|
These numbers should not be taken to be precise. The key point here is that risk reduction is already very high and that the addition of PreCheck does not change this desirable condition much one way or the other. However, insofar as the numbers can be taken seriously, they suggest that PreCheck most likely slightly produces a risk-reducing benefit.
This finding proves to be remarkably robust: when we change the assumptions in the model, the results vary only modestly. Thus, a risk-reducing benefit remains even if three-quarters of passengers are directed to PreCheck. The worst-case is to assume that passenger selection for the two security lines is no better than random (50/50). In this extreme case, the overall risk increases, but only slightly.
Potentially, the reduced deterrent effect of PreCheck might encourage ‘adaptive adversaries’ to bluff their way through the check-in process by somehow successfully presenting themselves as low risk to authorities. However, the risk-reduction of PreCheck still remains a benefit even if the likelihood of an attack by a terrorist assessed as a low risk passenger increases by 1000.
Overall, programmes to randomly direct some PreCheck passengers to the regular lines or to direct regular passengers to the PreCheck lines, particularly at times of high congestion, have little effect on the risk-reducing capability of the programme one way or the other. The key message, again, is not in the precision of the numbers, but in the fact that they don’t change much one way or the other when the parameters are changed, sometimes quite drastically.
The Economic Benefits of
The PreCheck programme does successfully reduce overall screening costs. Estimates suggest that these could easily be in the range of $100 million per year.
PreCheck also provides a very substantial additional benefit by improving the passenger experience. The Global Business Travel Association in 2016 found that it increases the numbers of satisfied business passengers by 12%, and for many it “makes for a better business travel experience”. There is, of course, great financial benefit to airlines if more efficient and faster screening reduces wait times because this leads to high passenger satisfaction. PreCheck clearly does so: a paper recently presented at the Transportation Research Board annual meeting reported that the median wait time for regular screening was 8.9 minutes but only 2.4 minutes in the PreCheck lanes.
In addition, a 2012 study published in the Journal of Air Transport Management found that reducing wait times from 10 to 5 minutes increased airline market share by 1% for a large airport in the U.S., something that could generate an additional $2.4 billion in U.S. airline revenues. Moreover, all businesses pay special attention to regular customers, and PreCheck is likely to be especially pleasing to the passengers the airlines most treasure: frequent flyers.
Security delays also inflict considerable costs on the economy more generally. A recent study conducted for the U.S. Transportation Research Board of the National Academies concludes that passengers value their time during check-in and security screening at $35.60 per hour, and recommends that this figure be used in cost-benefit analyses for government and private transportation projects and policies. If we apply that number, if PreCheck reduces wait times by a modest five minutes, and if 50% of passengers are approved for that programme, there would be savings of $1.2 billion per year in passenger time.
In total, the combination of reduced screening costs and enhanced passenger experience generates a co-benefit of over $3.5 billion per year.
Under a wide variety of assumptions, the PreCheck programme increases risk reduction, making air travel, if anything, safer from terrorist attacks. Moreover, the benefit of the programme is very substantial: by greatly reducing checkpoint costs and by improving the passenger experience, it generates a benefit of billions of dollars per year. TSA PreCheck thus seems likely to bring efficiencies to the screening process and great benefit to passengers, airports, and airlines while actually enhancing security. Our risk-based approach to TSA’s PreCheck programme might also be used to evaluate other risk-based passenger screening programmes – such as ‘Smart Security’ – that are being developed by the Airports Council International.
Mark G. Stewart is Professor of Civil Engineering and Director of the Centre for Infrastructure Performance and Reliability at The University of Newcastle in Australia. He has authored 5 books and has published more than 500 technical papers and reports. He has over 30 years of experience in probabilistic risk and vulnerability assessment of infrastructure and security systems.
John Mueller is a political scientist at Ohio State University and a Senior Fellow at the Cato Institute in Washington, DC. He has written over a dozen books, published hundreds of articles in scholarly journals and general magazines and newspapers, has been a John Simon Guggenheim Fellow, and is a member of the American Academy of Arts and Sciences.
Stewart and Mueller are the authors of Terror, Security, and Money: Balancing the Risks, Benefits, and Costs of Homeland Security (Oxford University Press, 2011), Chasing Ghosts: The Policing of Terrorism (Oxford University Press, 2016), and Are We Safe Enough? Measuring and Assessing Aviation Security (Elsevier, 2018).