For decades it's been one of the seminal American experiences. You're driving across the heartlands, flipping through the AM waveband on your car radio, sampling fire-and-brimstone preachers, rabid talk show hosts, and those weird local news snippets that remind you just how diverse this country is behind the façade of strip-mall, "Anywhere, USA" uniformity.

In contrast to Britain and the BBC, American radio has always been commercially driven: a station lives and dies by advertising. (The exception here is public radio, but even that only gets by with the help of periodic on-air fundraising drives that can be even more irritating than advertising.) But now many of these AM stations are at risk – and with them a slice of the country's history.

AM broadcasting, or amplitude modulation, to give it its technical name, is how radio began in the early years of the 20th century, when people built crystal sets and the first licensed US radio stations saw the light of day. AM became the vehicle for radio's golden age between the First World War and the advent of mass television in the early 1950s. In an America far more rural than it is now, AM radio was the main source of evening entertainment for tens of millions of Americans. It kept them in touch with the world too, relaying everything from FDR's fireside chats to stock and crop prices, from opera recitals to commentaries on far-away baseball games and boxing title fights.

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Albeit to a diminishing extent, it does a similar job today. The smaller stations are pillars of their community, broadcasting high-school football games and local news. Some of the larger ones are regional landmarks. KDKA, which claims to be the first station to have received a licence in 1920, is a Pittsburgh institution. Then there's KMOX, the "Voice of St Louis", from which there's no escape when I go to stay with my in-laws in southern Illinois, just a few miles away on the other side of the Mississippi river.

Or take WLW in Cincinnati, where I went in late 1994 to do a story about the rise of conservative talk-radio hosts who had contributed no little to that year's stunning mid-term defeat of their Democratic bête noire Bill Clinton. There the poacher turned prey as I was grilled on air by one of the most celebrated of their ilk, Bill Cunningham, as a strange interloper from foreign parts. Almost 20 years on, Cunningham's show is going stronger than ever.

These days short wave and satellite radio have shrunk the world but, in radio's interwar golden age, America's big AM stations did that too, none more so than WLW, which still likes to call itself "The Nation's Station." AM radio can travel remarkable distances at night, when the sun's rays no longer hit the ionosphere and radio signals reflect better, especially if they use a powerful transmitter.

These days, WLW and other major AM stations are limited to 50,000 watts. But between the wars the Cincinnati station broadcast with a 500,000 watt transmitter, the only one in the US to do so. As a result WLW could be heard at night virtually everywhere east of the Rockies and, it was said, as far away as western Europe. Even now, if the stars are properly aligned, you can follow a Cincinnati Reds baseball game from the plains of New Mexico, 1,500 miles away, courtesy of WLW. And why not, I might add? Other than cricket, perhaps, no game is more suited to radio commentary than baseball, with its rich traditions and constant pauses in the action.

But radio stations like WLW are becoming the exception. While half of the country's top 10 radio stations, measured by advertising, broadcast on AM, many of the others are in trouble. The glory days are long gone. Over the past three decades, AM's share of total radio listeners has fallen from around 50 per cent to just 15 per cent. In the 1970s, two thirds of all stations used AM. Today it has long since been overtaken by FM and, of modern-day America's 15,200-odd stations, AM accounts for only a third.

The reasons for the decline are several. One, of course, is the rise over the past half-century of FM broadcasting, which offers better fidelity and less interference – which is why almost every music station is now FM. Then there's satellite radio, and the growth of the internet. Above all, though, America is growing ever-more urban and high-tech. Tall buildings and power lines interfere with the AM signal, while today's proliferation of electronic gadgets means a host of new signals cluttering the radio frequencies. When reception worsens, listeners notice, advertisers go elsewhere and stations die.

"Well, so what?", you may well respond. Progress is an unsentimental, Darwinian business that inevitably generates casualties: why shouldn't AM radio go the way of the abacus, the town crier, the video-cassette and, quite possibly for that matter, the newspaper? There are plenty of more efficient alternatives and, if people are so attached to AM, let them switch to digital receivers.

"Not so fast", say supporters. They argue that AM, which is simpler and cheaper, is of special value to minority broadcasters. No less important, it really comes into its own during an emergency. Ice storms, tornadoes and hurricanes can knock out power and mobile networks, but not battery-operated radios. AM signals travel further too, enabling a greater number of people to keep in touch. Most importantly of all, the AM industry has one supporter that really matters – the FCC, the federal agency that regulates radio, television and cable.

Last month, the FCC published various proposals to revitalise AM radio. They include looser limits on transmitter power at night and an end to regulations that made it difficult to install new equipment. AM stations will also be allowed to use spare slots on the FM spectrum to boost reception in urban areas. If they work, every devotee of driving around America will raise his voice in thanks.