Surveillance technology is a science with more than a few terrifying applications to it. The very sci-fiety of it all is almost astounding. New tech can recognize a person out of a crowd based on their walk, or the shape of their elbow. Face-scanning and body odor identifiers might be able to single out people with sinister motives. Biometrics, micro-expressions... it's all there for identification.

Human gaits, for example, can provide a lot of information about people’s intentions. At the American Army’s Aberdeen Proving Ground in Maryland, a team of gait analysts and psychologists led by Frank Morelli study video, much of it conveniently posted on the internet by insurgents in Afghanistan and Iraq. They use special object-recognition software to lock onto particular features of a video recording (a person’s knees or elbow joints, for example) and follow them around. Correlating those movements with consequences, such as the throwing of a bomb, allows them to develop computer models that link posture and consequence reasonably reliably. The system can, for example, pick out a person in a crowd who is carrying a concealed package with the weight of a large explosives belt. According to Mr Morelli, the army plans to deploy the system at military checkpoints, on vehicles and at embassy perimeters.
...Another programme run by the Human Factors Division, Future Attributable Screening Technology, or FAST, is being developed as a complement to Project Hostile Intent. An array of sensors, at a distance of a couple of metres, measures skin temperature, blood-flow patterns, perspiration, and heart and breathing rates. In a series of tests, including a demonstration last month with 140 role-playing volunteers, the system detected about 80% of those who had been asked to try to deceive it by being hostile or trying to smuggle a weapon through it.

A number of “innocents”, though, were snagged too. The trial’s organisers are unwilling to go into detail, and are now playing down the significance of the testing statistics. But FAST began just 16 months ago. Bob Burns, the project’s leader, says its accuracy will improve next year thanks to extra sensors that can detect eye movements and body odours, both of which can provide further clues to emotional states.

So essentially what I'm asking is, why do I need to carry my driver's license with me everywhere?