My apartment overlooks a parking lot and a side street that has nicer parking restrictions than the main road, which means I hear car alarms not infrequently. (It seems like it is getting better, but it is possible that just like my friend who's parents lived on a flight path, I have grown more accustomed.)
I understand that the DC metro area has a high rate of car crime and that while a determined thief will always win (because they will tow your car, etc) that small things like a club can reduce your risk, particularly when there are better targets to be had.
However, I am not entirely convinced that car alarms fall into this category. After all I was once sitting innocently in my brother's car while he ran into the store when the alarm went off. He had taken the keys with him, which, on his car, apparently triggered the alarm after a certain period if the car could detect the presence of another person. I checked and saw that my brother had taken the keys, tried jiggling the steering wheel and triggering the locks, and when that didn't work I just sat there until he got back. A few people looked at me, mostly in annoyance. No one asked me what I was doing in that car. No one alerted my brother - or the other customers in the store - that the alarm was going off. My brother finished his business, walked back out, and went, "Oh, it was you."
So, had I had nefarious purposes, I suspect people would have thought I had accidentally triggered it and let me make off with the car.
Therefore, my unscientific conclusion is that car alarms are really only of interest to the car owner. But in cities where people might be parked a few blocks away or have loud neighbors, they may not hear it (unless they set it off themselves).
My suggestion, then, is car alarms that screech to your bluetooth headset. This is the second problem (although really it's more of a pet peeve, but it's my blog and therefore my peeves are problems). People (often guys) seem to like wearing their bluetooth headsets. I have a theory that it is people who secretly wish for more accessories in their lives, but really, it would make far more sense to wear that headset to the bathroom and to school plays and such if you could tell people it was in case the car alarm went off.
This would also assist in scenarios where people's car alarms might be set a touch to sensitive, because I feel certain that if people knew that their car alarm went off every time a truck rumbled by, they would wish to address that.