My early warning system, whether at home or on the road, is always a dog.
The dog doesn't have to be big, or even sound big or mean, the dog only has to bark. Dog barks, you know there's something out there. It might only be a coyote, but it's worth looking out to check, If I see another car or a person I don't like the looks of for any reason, I jump into the front seat and take off. I practice, and can get in the driver's seat and have the car in gear and rolling in only few seconds, not nearly enough time for anyone to get close enough to smash a window, because the dogs will alert before anyone gets very close.
of course, not everyone wants a dog! Failing that, the recording someone mentioned might make a person intending harm slow down enough for you to drive off.