The device is meant to work by knowing where it is, and what the speed limit is. The same way the Nissan GT-R knows it's on a track and removes the speed limiter, this would be the reverse: the car would know it's in a 30-MPH zone, and would work to keep the driver from going any faster. It's not I, Robot (above) -- although we'll say again: yet.
In the beginning, the system would only vibrate the accelerator pedal, or make a beeping noise to let a driver know he's speeding. At this point it doesn't look like there are any plans for the system to take control of the car, but one would have to assume that such ideas are being debated, and are on their way.