There is an email that has been circulating that claims that a car horn alarm can chase away bad guys.  All you have to do is activate it from your remote. Take your keys to bed with you and when you hear a noise, activate.
But is it really a good idea?
The answer from -- don't stake your life on it. For one thing, in urban areas, car alarms are so common that most people ignore them. Even burglar alarms are ignored by both neighbors and burglars. The advantage burglar alarms have is that they can often wirelessly call help.  A car horn will definitely annoy the neighbors, who will plan to speak with you -- tomorrow.
If you hear a noise inside or outside your home, crime experts advise calling 911.  Better to keep a telephone next to your bed than your keys. According to Snopes, the car alarm has prevented crime in the past, but the results were not stellar.
In 2000, a man attacked a woman leaving a day care center.  He was driven off when the woman activated her car alarm, but by that time she had been stabbed nine times.