That movie got me thinking, because it makes a number of claims about car alarms that jive with my personal feelings about them. They're noisy, obnoxious, tend to go off at night, and are almost universally ignored by passersby, people in nearby houses, and the car owners themselves. Despite these flaws, having one is almost default for new automobiles, and you even get a discount on insurance if you have one.

So the question is: Do car alarms actually reduce the incidence of theft of or from automobiles in which they are installed? If so, by how much? Whether or not the reduction (if any) is worth the hassle and noise will, of course, be a matter of opinion.

Edit: I'm not, of course, referring to car security systems in general. I'm only questioning the usefulness of the loud noise making aspect of them.

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    My guess is it would help deter car hoppers, but not more serious criminals. In residential areas, a popular crime among teenagers and younger criminals is testing the doors of cars parked near the street and grabbing whatever they can in a short time. A car alarm going off would send them right off the block.
    – Alain
    May 26, 2011 at 15:33
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    Anecdotally, when I was awakened in the night by car alarms, I'd be hoping the car molesters would either drive the car off or destroy it adequately to stop the alarm. May 26, 2011 at 15:41
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    All you need to do to defeat a car alarm is a minor fix - lift the rear end by 45 degrees. I saw a documentary about it once (boards.straightdope.com/sdmb/showthread.php?t=539237)
    – user5341
    May 26, 2011 at 16:42
  • @DVK so you're saying we need to develop some serious upper body strength. Good tip. :)
    – JYelton
    May 27, 2011 at 15:30
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    @DVK: I think the purpose of said alarm "feature" is to turn off the alarm when the car is being towed. However, I've personally seen many car alarms being activated while in the process of being towed. So this appears to be useless. Also, 45 degrees is pretty outrageous - I don't think that any of the tires would be touching the ground at that point, so the point of the car on the ground would be a bumper.
    – Ernie
    Nov 17, 2011 at 21:13

1 Answer 1


Audible car alarms do not have a significant effect on car theft.

The insurance data are unequivocal. In 1997, the non-profit Highway Loss Data Institute (HLDI) surveyed insurance-claims data from 73 million vehicles, to see which devices could prevent theft. Looking at cars from many different model years, across the country, the study concludes that cars with alarms "show no overall reduction in theft losses" compared to cars without alarms.

Paper: "Alarmingly Useless, The Case for Banning Car Alarms, in New York City", by Aaron Friedman, Aaron Naparstek & Mateo Taussig-Rubbo

There are a variety of antitheft and tracking systems on the market with costs ranging from basic audible alarms costing $50 to sophisticated tracking systems with $30 monthly fees. Unfortunately, the effectiveness of many of these devices is questionable. The sensitivity of audible alarms to touch or movement, for example, provokes a “boy who cried wolf” reaction. When a car alarm goes off, people tend not to react because the alarms activate so frequently for reasons other than actual theft. HLDI studies show no overall reduction in theft losses for vehicles with such alarms.

Article: Highway Loss Data Institute, "Insurance industry analyses and the prevention of motor vehicle theft," Business and Crime Prevention (Marcus Felson and Ronald V. Clarke, eds.), pp. 283-93, Monsey, NY: Willow Tree Press, Inc.: 1997

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    [edited] I wonder if we could get a link to the studies this report references though? Nov 17, 2011 at 18:07
  • @John Rhoades: It is my understanding that the article written by the Highway Loss Data Institute was a direct statistical survey of insurance-claims data. Nov 17, 2011 at 19:57
  • That is why insurers can offer substantial discounts for those who use a full Cat 1 alarm and immobiliser, and for some marques a tracker is required to get insurance at all.
    – Rory Alsop
    Feb 1, 2012 at 11:48

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