The so-called e-Plate, developed by the British firm Hills Numberplates, is a license plate that also transmits a vehicle's unique identification via encryption that can be read by a small detector, whose output can be used locally or communicated to a distant host.

"RFID is all the rage these days," said Bradley Gross, chairman of Becker & Poliakoff, a law firm in Ft. Lauderdale, Fla., "but my fear is that this use of the technology is tracking at its worst."

The reason for the concern in the legal and privacy-rights communities is that e-plates may expand the ability of police to track individuals by the movement of their vehicles.

A single RFID reader can identify dozens of vehicles fitted with e-plates moving at any speed at a distance of about 100 yards. The e-plate looks just like a standard plate, but it contains an embedded chip that cannot be seen or removed. It is self-powered with a battery life of up to 10 years.

"Police will be able to track your every move when you drive," said Liz McIntyre, an RFID expert and author of the forthcoming book, "Spychips: How Major Corporations and the Government Plan to Track Your Every Move With RFID" (Nelson Current, October 2005). "What if they put these readers at a mosque? They could tell who was inside at a worship service by which cars were in the parking lot."

Indeed, the makers of the technology boast that the e-plates can furnish access control, automated tolling, asset tracking, traffic-flow monitoring and vehicle crime and "non-compliance." The chips can be outfitted with 128 bit encryption to prevent hacking.

The problem is people other than the vehicle's owner quite often are at the wheel.

"Will this, ultimately, stop terrorism?" Gross asked. "The occupants of cars change continuously. Terrorists can steal cars."

Similar technology already has been used in the United States, experts said.

"The technology side of this is readily available, as it is used in the high-frequency battery-powered transmitters in the toll road systems like Fastrak," said attorney Dave Abel, with the international law firm Squire, Sanders & Dempsey LLP, who was an engineer before coming to the bar. "To use the toll road, a user signs up -- providing name, address, billing info, et cetera, which is stored in a database. Each time they drive past the reader station they are billed or a credit is deducted from an account."

Security access points could justify the expense, but placing them even at key intersections may not be very practical, according to lawyers at Pittiglio, Rabin, Todd & McGrath in Costa Mesa, Calif., a spokeswoman said.

The cost of roadside readers is significant -- although the price per chip is estimated to be only 20 cents.

Some experts said governments already are using the chips embedded in tollway access cards without heed to privacy rights. In Texas, for example, tollway authorities have been "making printouts of the records of every time you pass through a toll booth, what time you passed through," McIntyre said. "The government hasn't established a privacy policy for this, and people are not being informed that they are doing this. This is an instance of Big Brother on the highway."