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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 '11 at 15:33
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. –  David Thornley May 26 '11 at 15:41
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) –  DVK May 26 '11 at 16:42
@DVK so you're saying we need to develop some serious upper body strength. Good tip. :) –  JYelton May 27 '11 at 15:30
@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 '11 at 21:13

2 Answers 2

up vote 11 down vote accepted

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? –  John Rhoades Nov 17 '11 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. –  Sean Vikoren Nov 17 '11 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 '12 at 11:48

Statistically, yes

Insurance companies will offer you a lower car insurance if you have an alarm fitted. Since their pricing strategy is driven by accurate statistics, it is safe to assume that cars fitted with an alarm are slightly less likely to be stolen.

From confused.com (a site specialised in insurance brokerage)

Alarms, immobilisers and trackers can all help you to get cheap car insurance cover:

Fitting a car alarm is a good way to qualify for a cheap car insurance premium. Car alarms are a good criminal deterrent to protect your car and valuables like your stereo. Once activated, the alarm will sound if the car is jolted heavily, if the doors are opened or the windows are smashed. The latest alarms also come with a pager to alert you if they are activated.


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And how large is the difference? Or how much less do you pay? –  johanvdw Sep 27 '11 at 8:47
Significant with a limited effect-size would be the statistical formulation –  johanvdw Sep 27 '11 at 9:05
As a person who's worked in insurance, I can't say I find this reasoning convincing. It ignores many OTHER reasons why an insurance company would give discounts to a premium. Large insurance companies have deals with other companies in order to cross advertise and it would not surprise me to see something like that for car alarms. I get a small discount for being in AAA. I could get another one from a specific agent for being in Mensa. Or having gone to a certain highschool. Not all insurance pricing and discounts are statistically rigorous. –  John Rhoades Sep 28 '11 at 13:15
@Sklivvz Argument from authority? Nope. I'm pointing out reasonable alternatives to your interpretation of the data that I have learned are possible due to my experience. Claiming I have to be right due to my experience would be a fallacy. Mentioning that I am aware of those possible alternatives due to having worked in the industry is NOT an argument from authority. –  John Rhoades Sep 30 '11 at 17:23
@Sklivvz In fact, if we're going to talk fallacies, I should point out that your answer is a case of Affirming the Consequence. The reasoning goes, if car alarms reduced car crime then they would reduce car insurance costs. Car insurance costs are reduced with car alarms. Therefor car alarms reduce car crime. This WOULD make sense if auto insurances relied ONLY on statistics to make all their decisions, but they don't. They're a business, and occasionally base their decisions on other ways of making money. –  John Rhoades Sep 30 '11 at 17:32

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