Even door locks can provide useful information for someone wanting details on the comings and goings of others. Not old-fashioned lock-and-key systems, that is, but “access control” systems based on codes, pass cards, or radio frequency identification (RFID). Years ago, for example, I had a biometric, voiceprint-based lock on the front door of my house in Cambridge, MA. Everybody had a unique code, of course, so I was able to use the system to see if my live-in girlfriend was coming home on nights when I was out of town. (She wasn’t.)

All these data surveillance systems certainly prove themselves useful from time to time, and increasingly they’re being used by parents and corporations to keep track of children and employees. I recently spoke with a computer forensics specialist who told me that he used the log of a card-key system to show convincingly that an employee suspected of visiting pornographic websites and trying to break in to corporate computers had actually been framed by someone in his company’s IT support group. The attack happened at 2:00 a.m., when the employee was home in bed; the IT person often worked late.

Any monitoring system can be defeated, of course. A child who doesn’t want her cell phone tracked can turn it off or “accidentally” leave it at a friend’s house. I can wait at the front door of an office until it’s opened by a coworker. And my wife can unplug her CarChip if she doesn’t want to be tracked. The CarChip tries to defend itself against this ploy by recording the fact that it was unplugged and then plugged in again at a later time, but it can’t tell you what happened in the interim.

That’s why I think the real use of these systems isn’t surveillance but self-knowledge. I want to know if I am routinely driving faster than the speed limit, or if I am gunning the engine and then hitting the brakes. That’s why I ordered a CarChip for my little blue sports car. If I ever do get in an accident, I want to have proof that I wasn’t at fault. Unless, of course, I was, in which case I expect this little Big Brother to get mysteriously lost in the confusion that follows.

Simson Garfinkel researches computer ­forensics at the Harvard Center for Research on ­Computation and Society.

CarChip E/X
Davis Instruments
$179