THEY may be considered by some to be corny, vulgar and infuriating, but they are also apparently a warning sign to steer clear.
Researchers have found that personalised number plates, rear window stickers and furry dice adorning cars indicate an aggressive driver behind the wheel who may be prone to road rage.
A study has shown that motorists who customise their vehicles are marking their territory and warning of a readiness to defend it against incursions into "their" road space.
The same rule applies to drivers of small family hatchbacks as much as to the owners of giant gas-guzzlers and 4x4s, the psychologists found.
The more such "territorial markings" were on display, the more drivers jealously guarded their space, they said. Those displaying many stickers were 16 per cent more likely to succumb to road-induced anger.
The researchers said: "Both the number of territory markers and attachment to the vehicle were significant predictors of aggressive driving.
"The mere presence of a territory marker predicts increased use of the vehicle to express anger. The number of territory markers predicted road rage better than vehicle value, condition, or any of the things that we normally associate with aggressive driving."
The study, 'Territorial Markings as a Predictor of Driver Aggression and Road Rage', was published in the Journal of Applied Social Psychology.
Hundreds of students between the ages of 17 and 43 were quizzed about their anger levels in various driving situations and their responses matched with the level of personalisation of their cars.
Personalised number plates are either loved and loathed by drivers, with more than three million sold by the Driver and Vehicle Licensing Agency since they were launched 20 years ago. They cost a minimum of 245, with a "1D" plate selling for a record 352,411 this year. A total of 1.47 billion has been raised for the Treasury, at the rate of about 250,000 a day.
However, according to polls, personalised plates are among attributes motorists most hate about other drivers. More than three-quarters thought those who bought them were "self-obsessed idiots". Bumper stickers are also increasing in popularity with "Baby on Board" and "My Other Car is a Porsche" among the most common.
The practice of hanging furry dice started in the US in the 1950s, originating from the lucky dice hung by American pilots in their cockpits in the Second World War. Although their popularity has declined they are enjoying a retro comeback.
The research from the Colorado State University received a mixed response from motoring groups in the UK, with experts calling for further studies.
Automobile Association president Edmund King said: "Drivers with stickers tend to relate more closely with their vehicles, so perhaps take any perceived infringements more personally. This can lead to road rage. The best advice is to pull back, particularly if the car in front has a sticker on saying 'Back Off' or 'Insured by Smith & Wesson'."
Dr Mark Sullman, a driver anger expert at Hertfordshire University, said: "Drivers who are more territorial in nature are more likely to indicate this by personalising their vehicles. In addition, they are more likely to respond in an aggressive manner when they experience anger on the road."
Dr Nick Reed, a senior human factors researcher at TRL, the former Transport Research Laboratory, said: "Most drivers feel territorial about not only their vehicle but the space around them as indicated by the phrases: 'He's right on my bumper' and 'That car cut right into my lane'.
"It is therefore believable that anyone who has taken the trouble to adorn or 'enhance' their car in some way may feel more attached to it and therefore more protective if feeling threatened or obstructed."
However, the Royal Automobile Club Foundation said not all motorists who decorate their cars should be considered potential road ragers.
Phil Gomm, its spokesman, said: "Our cars are our own personal space. They are an extension of us. Why not decorate and adorn them as we wish? I am sure the tens of thousands of respectable motorists displaying National Trust for Scotland stickers, for example, will take great exception at any suggestion they have road-rage tendencies."
Neil Greig, of the Institute of Advanced Motorists, said: "If you are driving close enough to read the bumper sticker, you may already be seen as being aggressive by the driver in front."
Motor insurer Swiftcover advised drivers to steer clear of personalised vehicles to be on the safe side.
Marketing director Tina Shortle said: "We have found no evidence of increased claims involving these cars – but perhaps that's because our customers are keeping away from them."
Drivers said the research was too sweeping in its conclusions. Susan Wright, an Edinburgh photographer with a year-old son, has a "Small Dude on Board" on her Seat hatchback, but denied it was a road rage indicator. She said: "We have it on our car to let drivers know we have a small child on board in case of any accidents. I don't think it makes me a bad driver or more aggressive.
"I think it would make you a better driver if it's a small child sticker as you wouldn't be willing to risk the child in the car by driving madly."
TOP TEN STICKERS
Out of my mind. Back in five minutes.
Rehab is for quitters.
I get enough exercise pushing my luck.
The more people I meet, the more I like my dog.
Apathy: I could take it or leave it.
That's not a haircut, it's a cry for help.
Excess is never too much in moderation.
I used to have a handle on life, but it broke.
Being "over the hill" is much better than being under it.
Errors have been made. Others will be blamed.