To supplement cell-phone tracking systems like Cellocate, transportation agencies are also installing additional electronic toll tag readers along major highways. In some cities where toll booths are common, radio-frequency tags are attached to cars. As cars pass the reader, it detects the tag and subtracts a set amount of money from a prepaid account. (See How E-ZPass Works for information on this type of system.)
These radio tags, or transponders, can be used to time vehicles between points in a freeway system. Unlike with a toll booth, drivers would not have to slow down for the reading device. They would merely drive past it. By analyzing a particular car's time between two points, a computer can determine the car's location and speed.
These tags and the cell-phone tracking systems will make it almost impossible for someone to travel undetected, which has raised privacy concerns about this new technology. Cell-Loc has said that it would not sell information about motorists' locations to advertisers. Other companies have said that they are considering selling the information.