It's not the most politically correct of impersonations but it's done with affection, its accuracy testament to the number of times Simpson heard black men of a certain age extolling the virtues of the 1955 Chevrolet Bel Air that Bohren toured in, his wife and family alongside him and an Airstream trailer hitched on the back.
With Bohren, a walking encyclopaedia of black culture, as his guide, Simpson visited the birthplaces of his heroes, such as Beltonia, Mississippi, birthplace of singer-guitarist Skip James. This, and living two doors down from characters like Miss Lily, a pint-sized toughie who ran the first gay bar in America and could regale him with tales of Rock Hudson and Danny Kaye's surreptitious visits to the Pussycat Club, all fed into Simpson's music.
When he returned to England after 15 years in America, the blues he had married to the British folk song tradition in his teens had greater depth, strengthening a fusion that had always felt natural to Simpson, even if others, such as Ewan MacColl, disagreed.
"I never thought the two couldn't be mixed together," he says. "And I didn't get this argument that MacColl and Peggy Seeger put forward that you had to sing the music of where you came from. It's all the same stuff. I'll sing and play what I like."
His first musical hero, at the age of three or four, had been Paul Robeson, whose singing of spirituals made him aware of music's ability to make you cry. When he got his first guitar at the age of 12, he instantly felt he'd found the vehicle that would let him deliver the kind of emotion he'd heard in Robeson.
Through the apprenticeship of local club floor spots and a first paid gig at 14, Simpson reached national prominence as young guitar slinger, turned professional at 18 and made his first album, Golden Vanity, five years later. The manager he had acquired was soon ditched, however, when the chance to work with the estimable June Tabor came along.
"My manager said, 'You can't back up other people, you're a star,'" he says. "I was 23, for goodness sake, and June even then had the most extraordinary timing as an unaccompanied singer. I learned a massive amount from listening to her, and basically trying to keep out of the way as her accompanist. At the same time, I think she was learning from me. It was a great partnership and we could have gone on to do more interesting things if record company machinations hadn't got in the way, as they often do. But I look back and think: we were top stuff."
Simpson has collaborated with Tabor from time to time since, as he has with pickers and singers including Jackson Browne, jazz guitarist Martin Taylor, Richard Thompson, Martin Carthy, Cara Dillon and Kelly Joe Phelps. But his best work is by himself onstage, filling in the background to the songs he's chosen to sing, and playing that exquisite slide guitar style that's grown richer over the years.
"I'm going to be 60 soon," he says, "and I've spent the past two and a half months recording a new album that I wanted to reflect my entire intake, if you like. There were songs, like Leonard Cohen's The Stranger Song, that I heard at 13 and didn't learn at the time and suddenly I find myself singing that and I'm shocked. It's funny how some songs creep up on you and you realise, that's a great song, why didn't I do it before?"
Having finished recording, Simpson is completing the liner notes, which he feels are a very important part – still – of the listening experience.
"We just moved our vinyl collection downstairs and I can get lost forever in these old covers," he says. "They were my internet. I learned so much from reading them and I love writing about where the songs came from and why I sing them, although nothing compares to getting out on the road and gigging.
"Having spent so long off the road – two and a half months is probably the longest break I've had from gigging in 40 years – I'm buzzing about getting out to play again."
Martin Simpson plays Sound House, Edinburgh, tonight and Tolbooth, Stirling, on Friday.