The number of people driving 80mph through a residential area each year would be about the same as it is today: approximately zero. There are far fewer psychopaths than you think, and psychopaths are largely undissuaded by laws.
If you're young and inexperienced, or want to flaunt your disregard for safety, a "Speed Limit 25" sign won't dissuade you either. It might even persuade you.
