With the roar of applause still filling the night air, the motorcade moves out. There's a howl of sirens, a metal gate springs open and eight black vehicles leap down a concrete ramp and onto the expressway. We barge through stop signs with our motorcycle escort, waved on by police with scarlet light-sabres. We speed over bridges and plunge through tunnels, the neon glow a smear on the windscreen, the sound amplified by the rain. It's completely absurd and really rather thrilling. U2 are "doing a runner." Boston's basketball arena to the airport in just over six minutes. Is that a good runner as runners go?

"That's a fantastic runner," the Edge confirms. "I'd give it...ooh, nine point two. Better than Barcelona where they drive at a speed that's actually life-threatening. And better than Italy where the cops bang on your roof with batons."

The Edge wipes the condensation from the window and peers into the blur of blinking lights. He shrugs self-consciously in a manner that suggests the whole thing's preposterous but, at their level, it's the only practical way they can operate. "To some extent, you gauge the degree of affection within a city by the quality of the back-up you get," he adds, professionally. "And we've had an amazing connection with Boston over the years. They've always looked after us."

Dave Evans has lived like this for nearly 30 years, a cycle of songwriting, recording and performance that started when he was 17. He's known no other life. And for the past 20 years he's operated at this kind of level, travelling with a team of three technicians and 60 crew in order to replicate as faithfully as possible the music he creates in the studio.

He was born in Essex to Welsh parents, moved to north Dublin at the age of one -- "massive identity crisis!" -- and is now 44 with three daughters by his childhood sweetheart, and another daughter and a son by his second wife, the band's former choreographer. He's helped sustain a formula that sells both records and tickets in every last reach of the world market. He's the unsung hero who orchestrates the sound of the greatest rock 'n' roll success story of our time, a band for which his old schoolfriend is largely the public face.

"Bono is still hungry and that's the reason the band's lasted so long," the Edge's brother Dik Evans told me. "Bono does his bit -- and that's critical -- but none of them do as much as my brother. Edge is basically the engine room, working every day in the studio to make the records happen. And they wouldn't happen without him."

The convoy grinds to a halt in that remote outpost of Logan airport reserved only for the owners of private aircraft. Small and shiny Lear jets are parked on the tarmac. New and sparkling Gulfstreams stand beside them. And there at the back, dwarfing them all, is a 60-seater Airbus 320 emblazoned with the violet and orange insignia of the Vertigo tour and the logo of the city's four adopted sons.

It was from Boston, famously, that the 9/11 terrorists departed -- on flights originally heading for Los Angeles -- so security is now unimaginably tight. But there is a special dispensation for the quartet who have just entranced the 20,000-seater Fleet Center. "Sir," the customs are reminded, "this gentleman walks right through."

America has adopted U2 and nowhere more so than in the city we're leaving. Boston has the highest concentration of Irish immigrants in the States, and a student population of nearly 400,000, and it was East Coast college radio, back in 1981, that first picked up on the music of U2. The Edge remembers playing a bar in Boston to just 300 souls, opening for a band called Malooga. When their support set finished, the entire audience left the venue. They were breaking America below the radar.

Twenty-five years later, those 300 were doubtless back to renew the acquaintance, but this time they'd brought 19,700 friends. The roar greeting U2 was deafening, especially from the Irish quarter. One person waved a banner announcing GOD'S COUNTRY. Another hurled his striped green football top over the barrier and the Edge put it on, while the singer stalked the outer limits of the catwalk. Bono looked back, astonished. "Nice shirt, the Edge." He turned to the crowd. "Great to be home with our tribe!"

New singles from an act in their third decade are usually met with polite applause -- or in the States, cries of "Do some old!" -- but U2 have engineered the impossible feat of still being regarded as contemporary. Rarely in a show are you conscious of being plunged back into the past. Their recent material is so strong you don't even feel the need to go there.

Every night an entertaining drama is built around the band's inscrutable architect. As the Edge plays a note cycle like the call sign in Close Encounters, Bono leans into the microphone. "This," he points stage left, "is the same sound as the Edge's spaceship made when it arrived in the north of Dublin. Larry and myself and Adam just stood there and stared. A door opened and out came this astounding-looking man. Larry said: 'Who are you?' and he said: 'I am the Edge.' And Adam said: 'Where are you from?' and he said: 'The future.' And I said: 'What's it like?' and he said: 'It's better!' "

Half an hour later comes the supreme piece of theatre. Bono asks the crowd to hold up their mobile phones in a digital reconstruction of the Great Cigarette Lighter Scare of the 1970s -- in fact, a cunning ruse to then flash them the number of the One campaign for the eradication of Third World debt so they can text their support. Around the amphitheatre, on all six levels, thousands of pale blue lamps twinkle in the heavens. Everyone, even the band, appears stunned by the spectacle. "The Edge," Bono wonders, "is this your galaxy?"

The following afternoon, the Edge is to be found in a 12th-floor apartment in SoHo stirring an iced tea and gazing out to New Jersey through a wall of glass.

He is good company -- thoughtful, quite serious and very dry at times: "What I'm most proud of is my humility," he tells me at one point. His perfectly constructed sentences betray the intense powers of concentration required to operate the gigantic sound system reproduced on stage by, effectively, just a trio with a singer.

"Our music has no roof on it. It wasn't inspired by the juke-joint blues mentality, it was coming from a different place. We're playing to our strengths, and our strengths in some ways are to keep it very stripped down and simple."

But the trick, presumably, is to create a sense of intimacy, even when playing in an enormous space.

"Well I've seen bands in small venues and they can't communicate and I've seen stadium shows where everyone is completely unified, so it's not really about the size of the venue. It's largely about the songs. I saw Bob Marley in Dalymount Park, in Dublin, in '79, his last tour I think, and it was just unarguable. I've seen Springsteen on a couple of occasions where it's been unbelievable, that connection he's been able to create. I never saw the Clash in a big venue but they were one of the great live acts of all time, as were Stiff Little Fingers.

"I saw the Waterboys in the Top Hat, in Dun Laoghaire, around the time of The Whole of the Moon, one of those nights where it just went off -- the intention, the will, the wish to communicate, to get across to a crowd. Mike Scott was a great talent. It's not about internal performance, not about trying to maintain your cool. All that left me with an instinct about what's required to put on a great concert, where there's never a dull moment in the night. A lot of it is pure theatre. Springsteen has theatre. Jimi Hendrix had theatre. The Clash had theatre. But if that's all it is, then it loses out. You have to have an aspect of spontaneity where you really don't know what's going to happen. A jeopardy. An interaction between the performer and the audience."

How do U2 write songs?

"You start by being the new generation that establishes its identity by rejecting the previous generation, that's part of the cycle. But in the end you keep writing till you get close to what you think is the best song in the world."

Which for you would be what?

"Different things for different times. We'd always go to different people in our heads. It could be Bob Marley. It could be John Lennon. It could be the Clash. Early on it was probably the Fall, the Bunnymen, Magazine. All those influences are there at the back of your mind when you're working on something, and you kind of jump off those influences. But we've never put anything out because it reminded us of somebody. In the end it's got to be something unique.

"I thought 'Out of Control' was too like the Skids but in the end it sounded like us. 'Running to Stand Still,' I was worried it sounded too much like Lou Reed. The first version of 'One,' which I did with just an acoustic guitar and a piano, I thought sounded too much like John Lennon. Everyone else was saying: 'You're going to have to kick it somewhere else.' Then Brian [Eno] and Danny [Lanois, the co-producers] came in and gave it another twist and I came up with the electric guitar part."

"I gave it a tweak," says Lanois, "and I put that Les Paul mantra line on it. Edge is smart enough, they all are, to let ideas come from any part of the studio. They've had a number of very beautiful songs based on four repeat-chords, but he is the master of the riff, one of the great musical forces. He's the driving wheel, pretty much in charge of what happens harmonically."

"I said let's knock it around for half an hour," recalls the Edge, "and we went back to the main room at Hansa -- this huge room, a fantastic 1920s ballroom in Berlin; Bowie made Low there, Nick Cave's recorded there. All four of us went in, I started playing the chords, Bono got on mic and got that idea of the 'one' chorus pretty much in the first improvisation, and it had an amazing emotional feeling. It was pretty much there in three hours.

"Ultimately, if rock 'n' roll is going to be around in 500 years' time, it's going to be because people are still writing great songs. Bob Dylan was quoted as saying: 'I can't write those songs any more, I can't write like I did in the Sixties.' I'm not sure that I would agree with him -- though he would know better about himself than I would -- but our thing is always changing. We are still learning as a band after 25 years and that's about as good as it's ever going to get."

He talks about the score to the Spider-Man musical he's writing with Bono, intended as a Broadway production in two years' time. And about his wife and how she says he's insufferable for the first month after he comes off tour -- at eight every night he gets this tremendous burst of adrenaline. But mostly he talks about his relationship with three other boys from Mount Temple Comprehensive and the luck, hard work and delicate negotiation that's allowed their relationship to survive.

"Not everything Bono does would I necessarily agree with, but you've got to acknowledge that everyone's got their own particular opinion. We don't like all that the others do, but it's the compromise that makes it work. Nobody has ever betrayed their initial commitment to the group and what's best for it. It's all about the songs. If it's great, it's great in spite of us, not because of us. I honestly still think of us as four chancers from north Dublin."

The three other chancers get a little emotional when asked to shine the light on the engine room. "Edge, when I met him, always had this otherworldly quality," Adam Clayton reflects, "along with amused, detached cool. My impression now is exactly the same."

"In nearly 30 years," Larry Mullen says, "I've learnt never to underestimate him. On any level. His dogged, relentless search for the perfect song, the perfect sound, the perfect idea. He possesses so many qualities I aspire to."

Bono's slice of poetry is delivered with a wolfish grin: "Beneath the stillness, the Zen-like mastery of arpeggios and perfectly chosen crystal notes, there is a rage, an explosive side, as I've learnt on more than a few occasions. Never pick a fight with a man who earns his living through perfect hand-to-eye co-ordination."

This is an edited extract from a longer interview in the current edition of The Word (www.wordmagazine.co.uk)


© The Independent, 2005.