Not very far at all. A central road runs through Sweetwater, dividing the town in half; to the West, it leads to Gaithersburg, to the Northwest it leads to the Sweetwater Atomic Energy Plant, and to the east it leads to the rural outskirts of the town, including the stable. Characters can go maybe a mile past the stable or past the Atomic Energy Plant before they encounter the invisible force detailed in the FAQ.

As they get closer to the boundary they will have to depress the pedal more and more until eventually the motor of the car is not strong enough to push any closer and the wheels begin to spin in place. It's probably not a good idea to keep the gas pedal floored for tire reasons.