Through a system of soon-to-be installed receivers mounted on signs along freeways throughout the San Francisco Bay Area, the Fastrak system will soon be able to track individual cars, and could offer law enforcment officials the ability to track the whereabouts of known trouble-makers and other bad people of the future.
The Fastrak system was introduced into the Bay Area as a way to reduce congestion at bridge toll booths by letting people sail through the toll gates without stopping. A small transponder mounted on a car's windshield causes a remote receiver at the toll booth to automatically deduct the toll from the driver's account.
The new network of monitoring sites is designed to measure traffic pattern flows, and the current operations policy is that the system is not supposed to be used to track individual people, but there's no reason that this policy couldn't be changed in the future.
If you have a Fastrak pass on your windshield, you will be monitored as part of the traffic flow as you drive along Bay Area freeways. Participation is mandatory for all Fastrak-carrying vehicles -- there is no "opt-out" option, unless you want to stash the transponder in a portable copper mesh Faraday cage to block the signal when you're not near a toll booth. (Although an AP story quotes an anonymous source stating that the mylar bag that the device comes will block the signal, this seems unlikely.)