Hurrah! Brash Radio 5, which gives ordinary people a voice, is taking over from stuffy old Radio 4, nanny to the nation

Time was, whenever I took a cab to a friend's house, I'd find Radio 5 Live blaring in the bumping, grinding vehicle, and Radio 4 murmuring at my friend's. No more. Radio 5 reigns supreme everywhere I go.

In only four years since its launch, what was once a marginal station snubbed by the chattering classes as too populist has grown into an agenda-setter discussed at newspaper editorial meetings and Westminster chin-wags; and a talking point at dinner parties from Hampstead to Herefordshire.

Figures released last month showed Radio 5 Live achieving a record reach (over four million listeners). This success became public at the same time as Radio 4 sank to its lowest ever number of listeners (seven million), lowest total of hours listened and the lowest audience share. The dismal figures were blamed on the new controller, James Boyle.

The print media, led by the doyenne of broad-casting journalists, Gillian Reynolds of the Telegraph, poured scorn on his experiments in scheduling: he lengthened Today to three hours and shrank World at One to 30 minutes; he moved The Archers to 2pm and banished Yesterday in Parliament to long wave.

But the switch from 4 to 5 is not just about controllers or time slots. Radio 5 threatens to usurp Radio 4's status as the station of the nation because of the different relationship it has struck up with listeners. Where Radio 4 approaches its audience with the mixture of formal deference and jutting-jaw arrogance of a public schoolboy turned civil servant, Radio 5, with its frequent phone-ins and presenters' banter, engages in the informal and fast-paced conversation of the self-made man.

With their youthful presence and classless voices, 5 Live presenters - Peter Allen, Victoria Derbyshire, Nicky Campbell - are Identikit new Labour, enthusiastic professionals who convince us that this is a new era and anything is possible. Well before the Blair project trumpeted its mission against social exclusion, the 5 Live presenters shed the Olympian grandeur of Radio 4 stars and concentrated instead on facilitating our access to sports and current affairs. Its rolling news format and live broadcasting deliver Radio 5 listeners instant information, allowing them, if I may use "on-message" terms, to become stakeholders in the action. Frequent phone-ins further stress the participatory nature of this station: Jake from Hull gets to bang on about the Child Protection Agency and Jill from Leeds to share her contempt for fat-cat CEOs.

At a time when the arenas for social and political exchanges are shrinking, these verbal bunfights bring the sidelined centre stage, stoking their sense of citizenship by assuring them that their views count - or at least will be heard. And it doesn't matter where they live: Radio 5 has been truly devolutionary in its reach, winning an ever greater percentage of listeners in the Midlands, North-east and Scotland from South-centred Radio 4.

Conscious of the value that time has for its upwardly mobile audience, Radio 5 places a premium on news and sports, and shirks from the trivia that now pollutes the Radio 4 airwaves. Mindless quiz shows, mirthless comic turns, Matthew-and-Mum inquisition teams - Radio 5 remains mercifully free of these.

In any other country, this switch of the dial would serve as an interesting footnote in broadcasting history. In Britain, where television never fully weaned people off the "wireless", if you change the frequency, you change the nation.

For decades, Radio 4 has played nanny to the British, recommending readings and exhibitions and plays (Kaleidoscope); monitoring morals (The Moral Maze); and instilling daily habits - breakfast with the Today programme, drive to work with Start the Week, tea with The Archers. A shift in frequency will overthrow these patterns. It also threatens to dethrone national icons like Anna Ford, Jonathan Dimbleby and Robert Robinson, and replace them with Andrew Neil, Edwina Currie and Sybil Ruscoe - brash, irreverent and energetic lifeforces who bear little resemblance to the archetypal "English", with their ingrained sense of decorum, authority and fatalism. The roster of pundits used by 5 Live is equally different and anti-establishment: John Bird of the Big Issue and Derek Draper are among the voices that reflect a new image of Britain today. From their contributions we learn of a Britain in flux, where the old order is under fire and new questions are raised. Nothing is sacred.

As Radio 5 Live goes from being the station of the cabbie to the station of the cabinet, Britain stands transformed.