Global Positioning System (GPS) receivers that help drivers navigate unfamiliar roads are already familiar features on high-end cars. And the newest GPS technologies, recently tested in Sweden, tell drivers not just where they are, but even how fast they should be driving.

In tests conducted by the Swedish National Road Administration, researchers added customized receivers and warning systems to almost 1,000 cars. The digital-map databases built into the receivers included speed limit information for roads in several cities. In some of the cars, a black box on the dashboard flashed a light and emitted a warning noise whenever the driver exceeded the local limit. In others, the box displayed the speed limits in addition to the warning signal; and in a third group, the box was linked electronically to the gas pedal and imposed resistance whenever the driver was speeding. Drivers slowed down, and two-thirds of them reported they would like to keep the warning systems in their cars. Some test drivers did complain, however, that the technology led to "less joy in driving" and a "feeling of being controlled."