This is a kind of Elegy in a Country Petrol Station. (Forgive me, Thomas Gray.) All the deepest changes in landscape happen by chance. No one sat down and decided that, in English villages, the largest object in view should frequently be not the church but the big, bright, flat canopy of the petrol station (rival liveries for Esso, Shell and Texaco).

But that's the way it's happened, as anyone who drove around over Christmas and New Year knows. In England, though, the village church still wins hands down as a national emblem. Not so in the United States, where the gas station reigns supreme. There's an entertaining exhibition of American roadside architecture at the Building Centre Trust (Store Street, London WC1) until 17 January. The historian John Margolies opened it with a talk entitled 'Pump and Circumstance' (the march–tune Elgar, sadly, never wrote). He scours the US highways with his camera for gas stations built like flying saucers, teapots, Second World War bombers, icebergs, even a Greek temple. "If Aristotle had gotten gas," Margolies asserts, "he'd have got it there." One gas station took out a wedding licence and offered "Free marriages with purchase of five gallons or more of petrol." Margolies found a photo to prove it.

Some of these baroque creations are now official national treasures. John Margolies savours them as eagerly as John Betjeman savoured parish churches. They're how the West was won. After listening to Margolies, I now know that the first drive–thru gas station was built in Seattle in 1909. Historically this achievement must rival Seattle's other local innovations, Boeing aircraft, Microsoft, Starbucks, Frasier and Grunge music.

The philosopher of the gas station was Frank Lloyd Wright. "Watch the little filling station," he said. "It is the agent of decentralisation." In America, yes. But in England, the petrol station is the last prop of local life. It's the village shop; here you can glance at the Sunday paper's offer of "Britney's Personal Lesbo Photos" while edging towards the milk fridge. The sale of petrol seems marginal. Bunches of roses in crackly wrappers are bought by returning husbands, who peel off the price and the '20 per cent extra' label before they get back into the car.

The village couldn't manage without the petrol station, it thinks. If a family–run set of pumps ("No cards accepted") is bankrupted by bigger, company–owned stations or by cut–price offers in the market town's Sainsbury's, the local conservationists wring their hands. Many of them are second–home owners or down–sizing Londoners. But the retail trade has always been a cruel business, a bit like bull–fighting, with everyone taking turns to be the bull. Few customers care about the difference between one kind of petrol and the next — assuming there is any real difference. What matters are price and what the shop stocks. Or what the advertising looks like. No other product, apart from soap powder, is so blatantly ad–driven. "It's all the same gas," Margolies says, while treasuring Mobil's red Pegasus pump–top symbols. Vive la différence.

In Britain, the heftiest example of architecture–as–petrol–ad is the 1950s Shell Centre skyscraper, which looms up behind the Festival Hall. At one time, I refused ever to buy Shell petrol, except in dire emergency, because I objected to this building so much. In those days, like other fashionistas, I endorsed the Modern Movement rhetoric that Portland stone was always bad, and large plate–glass windows were always good. I've grown quite fond of this lumpy monolith since; I accept the architect Howard Robertson's argument that small office spaces don't demand big windows, and Portland stone weathers better than concrete. My reaction, I can see, was a bit harsh, and I buy Shell with the rest. Betjeman professed to be a great car hater, but he happily wrote Shell tourism guides.

In America, gas stations are among the most powerful social symbols, as John Margolies demonstrates. They combine, somehow, the scent of danger with the thrill of romance. Think of their symbolic role in Hollywood movies. At the end of Five Easy Pieces, Jack Nicholson climbs on a truck and leaves the girl : always the voyager. James Cain got it right, also, with a little help from Lana Turner and John Garfield, in The Postman Always Rings Twice (1946 version). Could this tale of sex and murder ever happen in a petrol station on the outskirts of Stoke Poges? You must be joking. Garfield, Turner and her Greek husband would sit down for camomile tea and a chat.