Advertising has taken over the modern world. We wear labels all over our clothes; you can't drive down a street without seeing an estate agent's board; and you're going to have to skip another one now to read about what I think is an ad too far.

I'm talking about the one at the back of the car you drive. You know, that sticker on the back window, the one that stays there for the life of the car advertising the dealer that first sold it.

It's a diabolical liberty, don't you think? Not only are you arguably paying over the odds, compared with our European and US cousins, for your motor, but you also have to drive around carrying some free advertising for the shark who pocketed your cash while mentally calculating his commission.

I remember a friend telling me years ago that when his father bought a new car, he turned up at the garage to collect it to find one of the stickers on the rear window. He kicked up a fuss and the dumbfounded dealer was forced to take it off.

And he was right. Why should car buyers put up with advertising the garage they happened to buy their car from? Think of all the people who will see the back window of your car over the years you own it. Talk about subliminal advertising.

So here's a little game for you. Keep a pad of paper and a pen in one of the cubby holes in your dashboard and every time you're stationary in a traffic jam jot down the name of the dealer on the sticker of the car in front of you. At the end of the month, add them all up. Now multiply that figure by £2,350, which is the cost of an ad on the back of a bus. That's the value of the advertising you've been staring at.

So while there might be not be such a thing as a free lunch, in the car industry there are plenty of free sandwich boards.