"Prostitution is a business basically. We cannot prohibit it, so we want to control it in favour of the sex workers and the population," said Michael Herzig, director of social services for sex workers in the city.

"Because if we do not control it, organised crime is taking over, and the pimps are taking over," he said.

Local authorities recognise that they have no guarantee that the new site will be successful but have worked hard to convince prostitutes to come and inspect the premises.

There are clearly defined rules. Clients must be older than 18, one to a car, and condoms must be disposed of immediately after use in the bins provided.

Behind the fence, a round track where clients make a choice and the women negotiate a price is flanked by the sex boxes - wooden frames that look like car ports where they can park and complete the transaction.

In town, clients regularly take prostitutes to a nearby forest or outside the city, where the sex workers regularly find themselves in dangerous situations, said Ursula Kocher, director of Flora Dora, a support network for prostitutes.

In the sex drive-in, prostitutes operated in a safe environment, she said, pointing out that in an emergency each sex box is equipped with an alarm button to alert a permanent security presence.

"The problem with the Sihlquai is that there are quite a lot of women, and it's a normal street in the middle of Zurich and all the neighbours and all these people living there had troubles because of the sex workers," Kocher said.

Violence against sex workers often follows from misunderstandings between clients and prostitutes, as the sex trade in Zurich is dominated by Hungarian Roma who speak only rudimentary German, she said.

In an article published in the Swiss tabloid , several prostitutes expressed reservations over the sex boxes, saying that this tightly controlled environment might intimidate their customers.

If some say they are ready to give it a try, others do not hide that they will simply take their business to the northwest of Zurich, once prostitution is no longer authorised in the city centre once the sex drive-in opens.

The proposed sex drive-in was approved by the people of Zurich in a March 2012 referendum, with 52.6 per cent voting in favour.

And the idea has a broad consensus among political parties, with only the populist Swiss People's Party (SVP) opposing the project.

"It will not work, either because the clients will not come or because the site will not be used by prostitutes," said Sven Oliver Dogwiler, a local SVP politician.

"It puts them in a cleaner space but one that is subsidised by taxes," he said.

The work cost 2.1 million Swiss francs (HK$17.6 million) and operating costs will be approximately 700,000 Swiss francs a year.

THE EUROPEAN EXPERIMENTS

• In Germany, the cathedral city of Cologne established a drive-in brothel in October 2001 with eight rooms, called "sex huts" or "performance boxes" into which motorists drive with prostitutes. Dortmund, also in western Germany, installed "prostitute boxes" a few months before Cologne. Twenty boxes operated for several years but in 2007 they became a nexus of criminal gangs from eastern Europe and were closed in April 2011. Bonn installed six wood-walled spaces in January 2011 in a car park near the city.

• Italy has several sex drive-ins. Prostitution is legal in Italy, but solicitation and brothels are not, so while the sites are tolerated, they have sparked opposition from local residents and the Catholic Church. In Pozzuoli, near Naples, two entrepreneurs opened a "Love Park" in May with 32 places for cars separated by blue tarpaulin curtains for privacy. The facility in a park was shut down by local authorities, reopened, and has now been closed again. It charged a minimum fee of €5 (HK$52) for two hours.

• In Belgium, where prostitution is not legal, but tolerated, the eastern city of Seraing wants to build a pedestrian-only "Eros centre" that would cover 21,500 sq ft to provide prostitutes with 34 rooms and a clean and safe environment. The design includes a discrete police locale and a 43,000 sq-ft car park, which in theory would be off-limits for sex.

• In Holland, where "window" prostitution has been legal since 2000, street-walking "Tipplezones" are tolerated in five towns, although Amsterdam closed such a site in late 2003. In the central city of Utrecht, almost 100 so-called "sex house boats" were recently closed, with the city's mayor citing concerns over human trafficking.