The plot to bring liquid explosives onto flights from London to the U.S. and Canada in 2006 prompted the Transportation Security Administration to institute a limitation of 3 ounces on all liquid and gel containers placed in carry-on bags.
This led to the now well-known and much maligned 3-1-1 rule for carry-on bags: 3-ounce containers inserted into a 1-quart bag for each passenger. The 3-1-1 rule has endured for 17 years. Since that time, airport security has moved forward strategically and technologically. The most significant strategic change has been the introduction of PreCheck in 2011, the risk-based system that makes travelers better known to the TSA, giving them access to expedited screening lanes at airport security checkpoints.
Currently, the TSA is deploying computed tomography (CT) screening devices for carry-on bags, which provide a more precise, 3D view of bags’ contents.
Given such enhancements and improvements, is the 3-1-1 rule still needed?
The United Kingdom decided that it is not and is taking steps to gradually phase out the rule. London City Airport is the first in the U.K. to abandon the rule and is using CT screening devices to scan carry-on bags, which can screen liquid containers of up to 2 liters, or around a half gallon, with greater precision. Liquid explosives have a different density than water, which the CT screening devices can detect.
To date, the U.K. government has said that where CT screening devices have been in place, there have been no security incidents. That is a nonsensical way to measure success.
The one blemish related to the U.K. decision is that not all passengers are the same from a security standpoint. Most are indeed benign. Some would even argue, correctly, that on any given day, all travelers are benign. Yet policies must be instituted to not only manage most days, but also outlier days. The CT screening devices provide an enhanced layer to lower risk and provide the necessary protection.
However, CT screening devices are not without limitations. They are subject to false alarms, which can slow the flow at security checkpoints, and false clears, which can lead to a security breach if it occurs with the wrong passenger. In the U.S., travelers going through screening lines have experienced slowdowns as TSA officers get acclimated to the new CT devices, even with the 3-1-1 policy still in place.
The U.K. is not proceeding blindly. It is also aggressively pushing for biometric facial recognition as a means to confirm travelers’ identities. Hence, if travelers are known to their security administration, then item restrictions, such as limitations on liquids and gels, could be relaxed.
Implementing a similar policy change at U.S. airports would require that more passengers be known to the TSA. This could be achieved in two ways.
One is by offering PreCheck at no cost to any traveler willing to undergo the necessary background check. The other way is to expand the use of biometric identity verification such as facial recognition, which would achieve a similar risk reduction benefit.
Such passengers should be permitted to abandon 3-1-1 bags. The passengers who remain unknown to the TSA would still be required to abide by the rule.
Some may argue that a known traveler to the TSA could still bring a liquid explosive through a security checkpoint and cause harm. This highlights why the rigor of the vetting process to become a known traveler or using biometric information should be key to relaxing the 3-1-1 rule, since the risks associated with such people are extremely low. The additional layer of security that CT screening devices provide would decrease any remaining risk.
So will the TSA follow the U.K.’s lead?
In the short term, no. However, the lesson learned is that responses to past threats need to be periodically revisited.
Getting past the 3-1-1 rule will require more passengers to become known to the TSA. The most significant headwind to achieving this using facial recognition is privacy concerns, which at least five senators have spotlighted, hoping to pause its expansion. If these senators are successful, the likelihood that the 3-1-1 rule will be abandoned for all passengers is small.
What the U.K. policy change is doing is pushing other countries to reexamine their liquid policy. The question is not whether a new policy is needed, but when and for whom a new policy should be instituted.
Sheldon Jacobson is a professor of computer science at the University of Illinois at Urbana-Champaign.