Safeway is installing electronic wheel-locking device on shopping carts at stores where the carts disappear at a rapid rate.

"We don't know how many carts actually are stolen," said Jeff Stroh, spokesman for Safeway's five-state Denver district. "All we know is that they are gone, and all the neighbors know is that they're abandoned in the alley."At $100 per cart, the cost of outfitting a typical supermarket with 250 to 300 carts adds up to $25,000 to $30,000, Stroh said.

In addition to Safeway, which has been using the wheel-locking devices at California stores with high cart-loss rates for at least a year, other supermarket chains are testing the system, said John French, inventor of an electronic caster made by his company, Carttronics. The cost is about $20,000 per store, he said.

"You just remove one mechanical caster from a regular grocery cart and pop on one of our electronic casters," he said. "It looks a little bigger because it has a little motor, a nine-volt battery and a radio receiver in it."

The other component in French's system is a double-insulated antenna buried in an inch-deep slot around the supermarket parking lot.

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If a customer tries to push a cart beyond the area defined by the antenna wire, the radio receiver in the caster activates the motor. It rotates a steel skid plate under the wheel, immobilizing the cart.

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