Michael Lodberg Olsen rests his buttocks on the board that serves as a bed in Sexelance, his mobile sanctuary for street sex workers, and begins rocking back and forth, his eyes twinkling with amusement. “We thought of putting in something to stop this,” he says, as the converted ambulance jumps up and down on its suspension. “But then we thought people might quite like it.”

Lodberg Olsen, a self-described social entrepreneur, brings a dash of humour to his projects. On the back door of the Sexelance, a sticker declares: “Don’t come knocking if the car is rocking.” He clearly relishes the terminology sex workers use for the services they provide: “‘Swedish’ is a hand job, ‘French’ is a blowjob, and ‘Danish’ is the real interaction.”

Lodberg Olsen’s intentions are entirely serious. A notice inside informs prostitutes that volunteers will call the police at the first sign of violence and encourages them to make contact if they are victims of human trafficking. “If you look at sex workers on the street, 42% have faced threats of violence, but in a brothel it’s only 3%,” Lodberg Olsen says, citing statistics from the Danish National Centre for Social Research. “There’s a lot of violence against street workers, so that’s what we are trying to deal with.”

Tonight the Sexelance is parked on Istedgade, a street tucked behind Copenhagen’s central station which is known for porn shops, strip clubs and prostitutes. Passersby peer in. A drunk clutching a can of Carlsberg rolls up and is shown the lubrication dispenser and tray of condoms, which come as oral, normal or extra large.

Three Romanian women in black puffer jackets and fur-lined boots, waiting across the street for the night’s first punters, have yet to notice it, however. “Really?! Come on! The customers no come there,” says Galina, 27, when she hears what the ambulance is for. “They go in the toilet, in the car park, in the car … noooo. Are you laughing at me? Come on!”

Susanne Møller, founder of SIO, which represents prostitutes. Photograph: Malin Palm/The Observer

Her friend Anna, 30, is less sceptical: “It’s very dangerous to work on the streets. Some people is crazy, some people speak very bad. If you made a place with rooms instead, of course the customers would go there.”

People in Copenhagen have learned not to underestimate Lodberg Olsen. Fixelance, Denmark’s first safe drug injection facility, which he launched in a similar ambulance in September 2011, helped change Danish drug policy. There are now five permanent facilities across the country and the original vehicle is exhibited at the National Museum of Denmark.

Illegal!, a magazine about drugs he launched in 2013 to give addicts a source of money beyond theft and prostitution, now has editions in London and San Francisco. “Pirate” boxes next to recycling bins for cans and bottles are a safer source for bin-divers who collect them for the returns, and have been replicated across Denmark.

“Maybe it doesn’t work, we don’t know, but at least we’re trying,” says Susanne Møller, a prostitute turned activist whose Sex Workers’ Interest Organisation (SIO) partners Minoritet, Lodberg Olsen’s organisation. “A lot of people say there’s a lot of violence on the street against sex workers, but very, very few do anything about it.”

While Denmark legalised buying and selling sex in 1999, it is still illegal to profit from other people selling sex, making it difficult for sex workers to rent premises, or employ drivers or security. Street Lawyers, a Copenhagen charity, has checked that Sexelance is doing nothing against the law. “If we were charging for entering Sexelancen, then it might be illegal,” says Maja Løvbjerg Hansen, one of their lawyers.

“It might also be illegal if a sex worker using the ambulance were shown to be a victim of human trafficking. We want to make room even for people in that situation to work more safely: there will also be information in the van on who to contact if you are there by force, so it’s not just a shag room. The purpose is harm reduction and to provide information.”

Susanne Møller and Michael Lodberg Olsen outside the Sexelancen on Istedgade in Copenhagen. Photograph: Malin Palm/The Observer

Shivering in Sexelance’s spartan interior on a cold November night, it’s hard to see the attraction. The hard board, which is too short to lie down on, was modelled on the bonnet of a Fiat Punto. “We are not running a brothel, it’s for street workers,” Lodberg Olsen says.

“They’re not used to a bed. This has got to be compared to a car or a telephone box.” Several prostitutes have suggested that the interior could be more comfortable. “They would like to have it nicer, with some more red colours instead of it looking very sterile, so that’s a possibility,” he says. “The key for us is to listen as much as possible to the sex workers’ needs.”

Soon he hopes to have a generator and a small heater. He is considering a remote switch so volunteers, stationed 10m away, can turn on a green light signalling that the facility is free. “It’s more important that we find the right site, and I’m not sure it’s here, or where we were yesterday,” he says.

The previous night, the ambulance was parked on the most established pick-up street in Copenhagen’s Vesterbro district, where the clients came by car. On Istedgade, they come on foot, but the location is too conspicuous. Møller suspects that the ambulance itself might be too eye-catching. “I looked at a picture of it today and I thought maybe it shouldn’t look like an ambulance, maybe it shouldn’t be so easy to spot. That may be a problem for the clients, because it’s too obvious.”

Lodberg Olsen predicts that it could be months before Sexelance draws in its first client, in stark contrast to Fixelance, which had eight people come in to inject heroin in its first three hours.

“I’m sure it will be used but I don’t know when,” he says. “We have to be patient, and the sex workers say that as well: ‘It’s a good idea, but be patient.’”

Even if the project fails, he and his partners will have learned that they need a permanent facility like the ones that have been successful in bringing Canadian prostitutes off the street in Vancouver.

“If it doesn’t work out, there will have to be a permanent place. It would be so much better if we had one of the small hotels around here.”