We just finished expanding the boundary for our dog's SportDOG In-Ground Fence system. These fences work by transmitting a radio frequency (RF) along a "boundary wire" that encloses the area where you want your dog to stay. The dog wears a collar that senses the RF signal and emits a beep/vibration when within a few feet of the wire and then, if the dog crosses over the actual wire, can emit a static correction or shock. You can set the level and the lowest level is very mild.
We live on 25 acres in a very rural area where many dogs have been hit by cars, simply because the roads are empty ninety percent of the time, leading to a false sense of security. Then a car comes zooming by at 50mph and the dog might not have time to get out of the way, and the driver might not see the dog. Fencing in a small area with a traditional fence was not a good option for us, either, for various reasons. We're glad we opted for the invisible fence; with proper training, our dog has learned the boundaries and is very reliable - staying inside them even with the greatest of temptations. He seems happy and relaxed when outside, not stressed about the boundary - he knows exactly where it is.