Raise your hand if you’ve ever tucked something into the waist of your pants because you have too much to carry. Now raise your other hand. Stand with your feet shoulder-width apart. The Transportation Security Administration (TSA) wants to scan you for suspicious items.

Thanks to advancing technologies , new ways to search people have become a permanent fixture on our near horizon. But they run into a few challenges. One is the Constitution . A second, even more difficult one is logistics.

Not for the first time , the TSA is working to implement search technologies at transportation hubs such as Grand Central Terminal in New York. One such technology is Pendar , which the TSA describes as “a hand-held video camera designed for short-range, point-and-shoot identification of visible residue levels of hazardous chemicals, explosives and narcotics.” Pendar uses Raman spectroscopy , which means shining a laser at something and analyzing the reflected light for signals of certain molecules.

Another is Thruvision , which detects anomalies in heat patterns coming from people’s bodies. Tucking a paperback into your pants at the sacrum blocks heat coming off your sweaty back, a sign you are sneaking something onto the airplane. (If Fabio Lanzoni modeled for the cover, you should be sneaking it!)

An annotated copy of the Constitution would be an anomaly to Thruvision, so let’s start with the challenge the Constitution poses. As I argued to the Supreme Court in a brief some years ago , using specialized tools, including trained dogs, to search people for chemicals, odors, and items is “searching”:

The basic rule, when government agents search, is that the search must be based on suspicion rising to the level of probable cause. The standard practice is to have that confirmed by a neutral magistrate who issues a warrant. None of this could be administered at a transportation hub.

The reflexive justification for warrantless searching is that it’s reasonable without suspicion or a warrant because threats to transportation are so great. In reality, they’re not, as dispassion will tell anyone doing transportation risk assessment. The argument also proves too much because transportation is not a uniquely vulnerable infrastructure. Anyone appearing where people congregate would be reasonably searched if open-ended location vulnerability were the rule. The Fourth Amendment was included in the Bill of Rights because of the colonists’ enmity toward general warrants that King George’s agents lorded over them. We don’t do that.

But when security is at stake, many people are willing to dispense with niceties such as constitutional rights. So we turn to another challenge: throughput.

About 750,000 people pass through Grand Central Terminal every day. Stopping even a tiny percentage to have their bodies “Thruvisioned” for hidden bodice rippers would be a huge task — and hugely expensive in terms of travelers’ time. The “false positive” problem looms large, as mascara bottles, books, and colostomy bags are vastly more common under people’s clothes than dangerous articles.

Then there is the problem of whom to stop. You can go to extraordinary lengths to prevent it, but this type of searching is ripe for aiming at “the black guy.” (Even if you randomize stops, the length of stops and the levels of courtesy or deference shown would undercut perceptions of neutrality and justifiably contribute to poor public perceptions.) If Grand Central indulged the idea of stopping people for these searches, it would have to delay and deeply offend hundreds of passengers per day to find a negligible quantity of dangerous or criminal items.

Using Pendar presents similar problems, if shifted in phase. For accurate readings, it probably requires stopping people too. If not, the vagaries of airborne particulates and residue could mean groups of people have to be pulled over (including “the black guy”) for superfluous searches. Recent veterans and participants in shooting sports would get special, wrongful attention. Pendar’s suitability for transportation security is highly dubious.

Timeless constitutional principles show these technological “fixes” for transportation security are broken, but the practical problems are probably the leading edge of the sword that kills them.

This article originally appeared in the AEIdeas blog and is reprinted with kind permission from the American Enterprise Institute.