Working Knowledge: In the Fast Lane; December 2001; Scientific American Magazine; by Mark Fischetti; 2 Page(s)

More than seven million Americans with electronic "tags" on their car windshields now cruise through deliciously vacant tollbooths while fellow drivers wait in lines. The convenience requires tollbooth equipment that interrogates a vehicle's tag with radio-frequency waves, validates the tag holder's account and deducts the toll from the account's prepaid balance-while imaging the vehicle with lasers and videotaping its license plates to catch cheaters.

All the action can happen in a few seconds because your account is not updated in real time. A database at a given toll plaza debits your balance there, but the network of toll plazas updates your central account only once a day. Violations are processed later, too.