To some people a car is just a set of wheels, but to an increasing number of Americans - especially those pinched by high petrol prices - it's also a mobile billboard, an opportunity to rake in advertising dollars by touting the virtues of Coca-Cola, Nestlé or Citibank.
Car-wrapping, as the concept is known, has been around for the past five or six years, but with petrol now selling for well north of $3 (£1.50)a gallon in many parts of the United States, and many vehicles remaining obstinately inefficient in their fuel use, it has grown into a full-blown trend.
In cities like Los Angeles or New York, it's not uncommon now to see the latest drama serial from HBO, the prestigious cable television station responsible for The Sopranos and Six Feet Under, being advertised on people's private cars. Quite what gets advertised on which vehicles depends on the habit of the individual driver and the demographic the advertiser is trying to reach.
The leading pioneer of car-wrapping, an LA-based firm called FreeCar Media, interviews each would-be car-wrap volunteer. A suburban "soccer mom" who ferries her children to school and sports games might be induced to advertise laundry detergent, say.
Sometimes FreeCar Media will install a GPS tracking device on the advertiser's car. Occasionally it will supply the vehicle for the advertising campaign (a hearse for Six Feet Under, say). More often it will fork out several hundred dollars a month in exchange for the bodywork-rental rights.
The client - who could be anybody from a national brand name to a local business - pays for the concept, the advertising exposure and the car-wrapping itself.
FreeCar Media says it has a database of about a million would-be car-wrap volunteers. It estimates around 150,000 vehicles across the United States now carry advertising - of which it is responsible for about 7,000.
Some of those vehicles, of course, are city buses or commercial vehicles, both of which have been carrying advertising for more than a decade. FreeCar Media, though, has been instrumental in expanding the concept of niche advertising on vehicles since it was founded in 1999.
Its managers specialise in "out of the box concepts" that have included, down the years, using cement mixer trucks to advertise Coca-Cola's Full Throttle drink, putting advertisements on port-a-potties, and helping companies recruit their own clients to carry their logos. The drivers are known as "brand ambassadors".
The concept has now spread to many more companies. A job recruitment firm in Phoenix, Arizona, called Jobing.com, offers its own employees bonuses if they agree to carry the company logo on their private vehicles. The company has a strict policy of good driving behaviour. One employee caught speeding had his car unwrapped before it could cause any unwelcome publicity.
Drew Livingston, the head of FreeCar Media, predicted this would be the wave of the future. "Employees want to make more money," he told an advertising trade paper.