One Saturday evening my wife said to me, "Were you driving a blue bus along Oxford Street this afternoon, about three o'clock?"

"Yes," I replied. "I was."

"Thought so," she said. "I was riding in the bus behind you."

"How did you know it was me driving?" I asked.

"Easy," she replied. "I could tell by the body language of the bus."

Exactly what this conversation revealed about my style of driving is debatable. One point is clear, however: buses always get noticed. Which is why they've been a popular advertising medium ever since the days when they were pulled along by horses.

Most London buses are red, of course, with an advertisement board stretching along each side between the lower and upper decks. Recently, though, entire vehicles have been given flashy new paint jobs to make the ads even more conspicuous. The blue bus in question was one such example: better known as the "Mary Poppins" bus, it was based for a while at King's Cross garage. And that's how I came to be driving it one Saturday afternoon (about three o'clock).

Most bus drivers take little notice of the adverts on their vehicles. Usually they just settle in behind the wheel and get on with their work, oblivious to whether they are helping to publicise liquorice allsorts, holidays in Oman or silky lingerie.

But of late it's been difficulty not to notice a certain message inscribed upon a certain bus. "THERE'S PROBABLY NO GOD," announces the slogan. "NOW STOP WORRYING AND ENJOY YOUR LIFE"

This is the so-called "Atheist bus" and its easy-going commandment was apparently sponsored by a select group of worthies with nothing better to spend their money on. Not to be outdone, various Christian groups have since responded by fielding their own buses, suggesting that there is, in fact, a God, and providing indisputable evidence based on the scriptures.

This game of theological ping-pong looks set to continue for some time, and should remain not only harmless but also irrelevant, just so long as the Jews, Muslims and Hindus don't join in.

Harmless and irrelevant are not words I'd use to describe another bus that's appeared on the scene during the past few months. This newcomer depicts the actor Daniel Craig in his guise as James Bond. Fourteen feet high he stands, at the rear end of a double-decker, looking immaculate as he loads new bullets into his handgun. He seems laid-back and nonchalant, emptying the spent cartridges on to the ground.

Obviously any driver who objects to being the purveyor of such messages is in the wrong job. Bus drivers aren't meant to have feelings. I've been driving buses in London on and off for 20 years and I don't particularly like the glorification of guns and weapons in ads for movies. But if I refused to drive a bus I'd be failing in my duties. After all, that's what we're paid for.

Meanwhile, almost every bus shelter in London shows some movie star or other pointing a pistol menacingly at the hapless passengers waiting in the queue. From Jodie Foster to Al Pacino, they've all got guns.

But the man of the moment is undoubtedly James Bond. After all, he's got his own bus. And the message is unequivocal: guns are OK. You can tell from the body language.

Magnus Mills is a bus driver whose novels include the Booker-shortlisted The Restraint of Beasts. His latest, the maintenance of headway, will be published by Bloomsbury in August