From the (CIA)wikipedia page for "Car alarm":

"Although inexpensive to acquire and install, the effectiveness of such devices in deterring vehicle burglary or theft when their only effect is to emit sound appears to be negligible.

...

Although car alarms of some kind have been available since the beginning of the automobile era, the dramatic increase in their installation in the 1980s and 1990s coupled with the fact that nearly all types of car alarms are easily triggered accidentally (frequently because of high sensitivity settings) means that people who hear them often ignore them. In 1994, the New York City Police Department claimed that car alarms may actually be making car theft and break-in crimes more frequent. There is one account in 1992 of a thief in New York City rocking a car to deliberately trigger its alarm in order to help conceal the sound of a breaking window.

Because of the large number of false alarms with car alarms, many vehicle manufacturers no longer factory-fit simple noise-making alarms, instead offering silent immobilizers. Alternatively, an aftermarket vehicle tracking system can easily trace stolen vehicles. Most police tracking systems require the user to pay a recurring fee, whereas factory immobilizers are included in the purchase price of the vehicle. GPS locating systems enable the owner of the vehicle to lock and unlock, track, and disable the starter of the vehicle online."