When people talk about Ed Ruscha, it isn't long before they're also talking about Los Angeles, the artist's adopted home town. Boxy cars and wide boulevards, deserted gas stations and lonesome illuminated signs: these are the things, they'll tell you, that Ruscha has made his own. "I like to think the California sun has burnt out all unnecessary elements in his work," says film director David Lynch. "[He is] the visual deus ex machina of what has become the most over-scrutinised city on earth," says novelist James Ellroy. "The coolest gaze in American art," said the late JG Ballard. Even Ruscha, who says LA has had less influence on him than they assume, admits that the city "leaks into" his painting. How, then, is his work going to go down in the Highlands? All that colour and light in the middle of a Scottish winter. Ruscha, who can be rather droll, looks at me over his cup. His eyes narrow. "I guess you could say this is where context comes into its own," he says, with a salty laugh.  To explain: Ruscha's work will shortly be seen in a series of Scottish galleries – in Thurso, Inverness and Helmsdale – thanks to a tour that comes courtesy of Anthony D'Offay, the London dealer who sold his art collection to the Tate and the National Galleries of Scotland in 2008. D'Offay, who generously sold more than 700 works for the price he paid for them, rather than for what they could have achieved on the open market, stipulated that the collection be divided into a series of rooms, each devoted to an individual artist, thus allowing it truly to be shared by the nation (different artists tour different regions simultaneously).  For his part, Ruscha was so taken by the idea that he swiftly donated a piece of his own, The Music From the Balconies (1984), to his "room". And now he is in London, telling anyone who'll listen that they should go visit, should they happen to be north of the border. "I like this concept," he says. "It enables people who wouldn't ordinarily see my work to get to know it a little. That really is something else."  We are drinking tea at a discreet corner table in Claridge's, which is a pity, because I would quite like all the social X-rays to get a good look at us. At 72, Ruscha (you pronounce it Roo-SHAY) is a devastatingly attractive man and I suspect he knows it, for all that he throws off a couple of jokes about his imminent mortality. (When I ask him, at the end of our meeting, what he is going to do with the rest of his time in London, his hand flies to his forehead and he says: "I thought for a minute there that you were going to ask me what I'm going to do for the rest of my time on the earth!")  He has a gravelly voice – the kind that invites you both to move your head closer to his and to keep your eyes firmly on his lips – and the slightly impassive manner of one who is used to women falling at his feet at openings. He also indulges in that very subtle form of flattery by which he will agree with almost any proposition I put to him. The luxuriant grey hair, the flinty eyes, the soft blue shirt: sitting with him is like sitting with an old-school American movie star, only with the distinct advantage that Ruscha talks (and so unpretentiously) about art rather than about his "method".  It's almost a year now since the Hayward's hefty retrospective of Ruscha's work on canvas, a show that prompted a huge outpouring of love for him in the press and among both the public and his celebrity collectors. ("A true American hero, the lonesome cowboy pointing a finger at our consumerist greed," said Jerry Hall.) Was he surprised by the strength of the response? "Well, it was a colossal project and I worried I would have some itchy kind of regret about it, that people might think: that's it, it's all over [for him]. Luckily, I happen still to be working, so I wasn't tricked into thinking that was the end of the story, even if other people were."  What compels him, after all these years, to go to his studio every morning? "Just that I can't retire because there's no other job for me." Is he there nine until five? "No, it's patchier than that, but I do still look at myself as a man who takes his lunch pail to work. I don't mean that literally; sometimes, I don't eat at all during the day. But I like the thought that I'm there to work. Of course, work takes different forms. I can spend two or three days without completing anything and it's choppy: it's filled with all kinds of irrationalities and stupid actions. I have some notion and then I drop it because something else comes along. I'm forever darting from one side of the room to the other. Sometimes, four days will go by and I've just kind of, like, piddled. Basically, though, I'm a creature of habit. It [art] is your life's commitment, as grandiose as that sounds. But it's also backed up by habit."  Ruscha is a midwesterner by birth. He was born in Omaha, Nebraska, and grew up in Oklahoma, where his father worked as an auditor for an insurance company. It was, though, apparent to him from an early age that he best get out. "I knew I wanted to be some kind of artist from about 12. I met a neighbour who drew cartoons and I had an idea I wanted to be a cartoonist – or something that involved Indian ink, at any rate. My father was quite traditional, but my mother was more open to the world. I got some encouragement from her. She was ready to take on these new possibilities. She introduced me to poetry. The trouble was, there didn't seem to be much room for that kind of thinking in Oklahoma. Something told me to go west."  At the age of 18, accompanied by his childhood friend, Mason Williams, the song writer and musician, he set out for Los Angeles. "I didn't leave out of anger. Afterwards, I still visited. But it was a pretty big deal. I was pursuing this new frontier. It was all so attractive to me: the vegetation, the sunsets, the lifestyle, the jazz. I'd read about Los Angeles and this fact stuck in my mind: that the city gained 1,000 new people every day. In 1956! A thousand people every day! I felt: I want to be part of that." What did he take with him? "Just an old suitcase. The journey took three days. It was like being in On the Road. We stayed in cheap motels. In those days, they called them 'trailer ports'. Ten dollars a night. It was an adventure."  Once in the city, Ruscha found himself a room in a boarding house and a part-time job as a bus boy in a restaurant, where he spent most of his time opening cans of tuna. Was he broke? "Yes. But you don't – or you didn't then – need much to stay alive. I remember it'd be saltine crackers for three days, until I got a little help from home." Even once he'd enrolled at the Chouinard Art Institute (now the California Institute of the Arts), he still had a job – unpacking crystal and ceramics for a Swedish importer – on the side. "There was no hope for any kind of big opportunity. I'm not saying it was hopeless. The big pay-off was to work as an artist and gain some shred of respect from your friends, who were also artists. But there was never any notion that you could make a living out of art. On the rare occasions you had a gallery show, and sold a little work, well, that was just gravy."  After graduating, he worked as a layout man in an advertising agency. Wouldn't it have made sense to move to New York, where there was a burgeoning and serious art scene? "A lot of people certainly felt like they had to go and live in New York. That was the centre. I loved it, too; it was exhilarating. But it was either too expensive or too tough. At any rate, something kept me away. Maybe I need [access to] wide open spaces."  He admired Marcel Duchamp, Kurt Schwitters, Man Ray and the futurists and clung to the idea that the brave new world that was opening up on the east coast would eventually make its presence felt on the west, too. "I just sort of snuggled in with that thought. But it was baby steps… I didn't have a game plan. I didn't feel I needed to be instantly understood or accepted. I just wanted to show my work if I could."  Ruscha was 35 before he could be said to be making a living, however scrappy, from his art alone. Does he think the struggle helped him to be a better artist? "It's easy to be romantic about it but, no, I think that's a myth. Struggle is not … essential." Of course, it's completely different today. "Students are promised vocational success. They come out of art school and they assume that if you have a gallery show in New York, it all sells out. I had lots of shows where I didn't sell anything and no reviews at all."  Ruscha has often said that he hit on what he wanted to do in his painting relatively early on and it's certainly true that, as he puts it, "a little silver thread" runs neatly through his 50-year career. Many of his paintings from the late 1950s and early 1960s, involve what is now thought of as his trademark: words. BOSS, OOF, HONK. HOT, RIP, STOP. Ruscha used words as linguistic readymades; he painted them not because he liked what they meant, but because he liked the way they looked, a legacy both of the time he spent in commercial art and a reaction against his abstract expressionist contemporaries Jackson Pollock and Willem de Kooning. And people really fell for these enigmatic non-messages. (Even presidents: his painting I Think I'll, which plays with the phrases "maybe" and "wait a minute", now hangs on the walls of Barack Obama's White House – a somewhat unreassuring thought, I find – and in July, during David Cameron's visit to Washington, President Obama presented him with a signed lithograph by Ruscha, Column With Speed Lines, chosen for its red, white and blue colours.)  "Learning how to set type, that had a big effect on me," he says. "But I also liked, in LA, all the misspelled words on signs and the homemade signs, like 'watermelon' [for sale]. I thought of it as a kind of folk art. Getting them down, painting them, is like making them official, glorifying them, putting them on stage. I guess that's what poets want to do: put ideas on stage. I settle for a single word."  His work was included, along with that of Roy Lichtenstein, Andy Warhol and Wayne Thiebaud, in the 1962 show, New Painting of Common Objects, curated by Walter Hopps at the Pasadena Art Museum – and ever since, Ruscha has been associated with pop art. Does he mind? "I felt it was a mistake that the popular world had hitherto not been accepted as proper subject matter for art. There's room for saying things in bright, shiny colours. You don't have to accept the so-called harmony of the Greek vase. We were responding to notions of common objects." He looks around the room. "Here we are in this place. How do we make something out of it?" He picks up a delicate-looking round of shortbread. "Maybe this cookie is the centre of this universe."  How much was he helped by his late friend Dennis Hopper, a man who managed, somehow, to straddle the worlds of art and Hollywood? (There exists a wonderful photograph of Ruscha by Hopper; taken in 1964, Ruscha is standing in front of a neon sign that says: TV RADIO SERVICE.) "Well, he had an enormous appetite for anything to do with art. He managed to rack up a good deal of… product [Hopper both collected and made art]. But he was very aware of art movements; he was always on the edge. It was inspiring to see. I hated to see him go." Did they have wild times together? "Oh, we had good times. He lived a harder life than me. Bigger ups, bigger downs."  Together – the two men had studios in Venice Beach – they watched the city change; paradoxically, it became both less sleazy and less glamorous – "Though people still think it's a good place to hole up, to hide." These days, Ruscha, who divorced Danna, the mother of his son, Edward, in the early 1970s then remarried her in the 80s, also has a house in the Mojave Desert, a place so quiet, his mother once told him: "You can hear your hair grow out here." But he loves it. Ruscha disdains computers and mobile phones, he doesn't watch television and has never, to his knowledge, entered a Starbucks. "I cherish the idea of being alone," he says.  It is at this point, appropriately enough, that a representative from the Art Fund, which is supporting the Artists Rooms tour, appears to tell me that our time is up. Before I go, encouraged by Ruscha's extreme affability, I ask him – not a question one would put to every artist – if he felt, when the Hayward mounted its retrospective, that his work had improved over the years? "No, I haven't got better," he says. "I've just gotten bigger. I'm talking square footage." He laughs, perhaps because I look slightly startled. "No, I meant that in a metaphorical sense, of course. But perhaps I'm just hoping. Perhaps I'm as locked down as I always have been. This idea of American culture, it's an old one. And the words and all that are just the tail end of an ancient tradition that began with man scribbling on a cave wall. I'm observing that these words, which sometimes represent objects and meanings, are made up of these squiggly little forms we call an alphabet." He looks almost apologetic. "It's another way of looking at things, that's all."  • This article was amended on 15 September. The original said Hot, Rip, Stock. This has been corrected.
As an exhibition of Ed Ruscha's work tours Scotland, the painter with the 'coolest gaze' in American art talks to Rachel Cooke