Almost everyone I know locks their car doors when they leave their vehicle for an extended time. However, I come from a country town and most people in that country town don't lock their car doors and rarely lock up their houses.

I moved to the city to live when I was 8 years old and have always wondered what the point is in locking your car doors? If someone can start your ignition without a set of keys they can surely, with even less effort, open your car doors.

Is there any evidence to suggest that locking your car door reduces the chance of your car being stolen or reduces the chance of a valuable being stolen from an out-of-sight area (glove box, car boot)?

Examples of the claim that people should lock their doors to prevent vehicle theft:

Tips to help you prevent vehicle theft: Close all windows and lock all doors before leaving your car unattended.

Lock your car. Approximately 50 percent of all vehicles stolen were left unlocked.

How To Help Prevent Vehicle Theft: Lock your vehicle every time you leave it, even in the driveway or garage of your home.

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    The big threat that locking protects against is casual theft. (After all, who wouldn't want my ten year old road atlas and the change in my ashtray?) But even a thief who can jimmy open my car door takes a bigger risk of being seen doing it. He may look for easier targets... Commented Aug 7, 2012 at 0:09
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    The doors protect the contents of your car, whereas the ignition protects the car itself. Can you provide an example of the claim that door locks protect the car, in stead of simply its contents?
    – Sklivvz
    Commented Aug 7, 2012 at 6:51
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    @warren Statistically you are wrong there, unless you can show that 50% of cars area left unlocked. If 90% of cars are locked and 50% of stolen cars area unlocked then leaving your car unlocked seriously increases the chance of theft. Commented Aug 7, 2012 at 18:28
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    @war Assume a parking lot with 10 unlocked & 990 locked cars. 20 cars are stolen, 10 locked and 10 unlocked, then 50% is very significant, as it's a 100% chance of having an unlocked car stolen versus a ~1% chance of having a locked car stolen. If instead we had a population of 400 unlocked & 600 locked cars, with 20 stolen cars, we'd have 2.5% unlocked & 1.7% locked cars stolen. Not very significant. But if we had 800 stolen cars out of the same parking with the same ratio, the chances would change to 100% vs 67%. So, one needs all the numbers to make sense of such a claim!
    – Sklivvz
    Commented Aug 7, 2012 at 20:05
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    I thought it was common knowledge, but apparently not. Most cities have organized groups of people, typically made up of teenagers, who pounce on neighborhoods and simply goto each car and try the door handle. If it's locked they move on, if it's not they take the radio and anything else they find. They can typically clean out several blocks in under 5 minutes which makes them very hard to catch. In these cases, if your door is locked you are fine, if left open then at best you've lost your radio.
    – Dunk
    Commented Aug 13, 2012 at 19:03

1 Answer 1


Research evidence in 2015 from 16 studies that have examined the impact of electronic immobilisation on vehicle theft in the United Kingdom, Germany, Australia and the USA show that electronic immobilisation has been successful in reducing vehicle theft with 15 studies reporting a positive impact on vehicle theft when compared with those with no immobilisers fitted.

These reductions have mostly been larger for temporary (recovered) vehicle thefts than for permanent (unrecovered) thefts. This may also have resulted in a reduction in young people engaging in vehicle theft. Although some studies showed there had been displacement towards vehicles without electronic immobilisation, this was outweighed by the reductions in vehicle theft observed overall.

Also based on analysis of data from UK by Graham Farrell et.al. in 2011, central locking and electronic immobilizers are the most effective pairing against theft of cars.

Cars with central locking plus an electronic immobilizer, and often an alarm, are found to be ‘SPF 25’, that is, they were up to 25 times less likely to be stolen than those without security. That impact is greater than expected from the individual contributions of those devices, and is attributed to interaction effects. Tracking devices are found to be particularly effective but rarer. Protective effects were greater against theft of cars than against theft from cars or attempts, almost certainly reflecting the difficulty imposed on thieves by electronic immobilizers.

Electronic vehicle immobilisation in Australia through Australian Standard (AS) immobilisers typically disables two or more electrical circuits (linked to either the ignition and/or fuel pump circuits) built into the engine management system.

Electronic immobilisers most commonly work through a small transponder in the ignition key that transmits a weak radio signal, broadcasting an encrypted code that is picked up by a receiver located close to the ignition lock. When the expected code is received, the electronic immobiliser is disengaged.

Examining vehicles registered after 1991, the theft rate for vehicles in 2000 was found to be 29.0 per 10,000 vehicles registered, compared with 52.8 per 10,000 for vehicles with a non-AS immobiliser fitted and 47.8 per 10,000 for vehicles with no immobiliser at all.

Referring to Rick Brown, similar results have been observed in the UK and Germany.

Similar results have been observed in the United Kingdom (Brown 2004; Brown & Thomas 2003; Farrell et al. 2010; Farrell, Tseloni & Tilley 2011; Lee, Wyndham & Fairman 2006; Webb, Smith & Laycock 2004) and in Germany (Bässmann 2011), where significant reductions in vehicle theft were observed following the introduction of European Union regulations requiring all new vehicles sold in Europe to be installed with electronic immobilisers from October 1998. In the United States, where there has been no regulation requiring manufacturers to install electronic immobilisers, such devices have taken longer to penetrate the vehicle fleet.

Referring to Cecil Adams, central locking has a Security Protection Factor (SPF) of 2.7.

A car alarm all by itself, according to Farrell and company, has an SPF of just 1.2, whereas central locking is rated 2.7. But an alarm plus central locking has an SPF of 5. Pile on the technologies and the multiplier effect becomes more pronounced. At the top of Farrell’s list of antitheft technologies, with an SPF of 25.4, is the following fourplex: central locking plus an ignition kill-switch plus a tracking device plus, yes, an alarm. Farrell calls this the ACET configuration.

These findings should be reviewed with a note of caution since Twitter security engineer Charlie Miller and Chris Valasek, director of security intelligence at IOActive have studied the schematics of the 20 cars in 2014 and found that the 2014 Jeep Cherokee and 2015 Cadillac Escalade were the most vulnerable to computer attacks, while the 2006 Ford Fusion and 2010 Range Rover Sport were the most secure. Among the other cars that performed poorly in the testing were the 2010 and 2014 Toyota Prius and the 2014 Infiniti Q50.

Manufacturers also continue to work with tech experts to highlight potential bugs in the network, and fund research to establish better security systems for their wireless systems. It’s also worth pointing out that each manufacturer has its own data and computer systems in its vehicles that the hackers will need to overcome. Just because one car has been compromised by hackers it does not mean that all models are vulnerable to the same attack.

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