Story highlights Sex workers will operate from garage-style structures with alarm buttons and guards

The move is intended to take prostitution off the streets of Zurich, says project leader

Some 30 to 40 women are expected to work at the site in Sihlquai each night

The women can get health checkups on site, as well as advice from social workers

In an attempt to reduce open street prostitution and to improve security for sex workers, Switzerland's largest city, Zurich, is opening "sex drive-ins" Monday.

The nine garage-style structures, located in Sihlquai, a former industrial zone in the city, are equipped with alarm buttons and guarded by security personnel to ensure the safety of the prostitutes. Customers are not allowed to leave the area with the sex workers.

"Prostitution has escalated in the city," said Michael Herzig, from the Zurich social services department. "It was done out in the open on the street and men harassed passers-by.

"It was noisy and dirty because men took the prostitutes to a side street and left used condoms on the streets. The drive-ins are more discreet and safer."

Herzig leads the project, which has cost 2.4 million Swiss francs ($2.6 million) to set up.

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"The project is extremely controversial. Prostitution is about morals and religion. But we leave these things aside and see it as a business. The humanitarian aspect is more important for us," said Herzig.

Around 30 to 40 women are expected to work at the site each night. Sex workers have to pay 5 Swiss francs per night to make use of the so-called "sex boxes," but customers don't have to pay an entrance fee.

The drive-ins are open daily from 7 p.m. to 5 a.m. and can only be accessed by car. Drivers have to follow a clearly marked route to get to the area, where customers and sex workers can discuss the services offered and agree on a price. The client can then drive into the allocated box, where the service is provided.

There are also showers, toilets, a kitchen and washing machines for the women to use, as well as an advice center where they can seek help from social workers. A gynecologist comes in once a week to offer health checkups for the women.

The city's social services department is running the whole operation and also offers crash courses in German and self-defense courses for the women on site. "All of this will enhance the sex workers' living and working conditions," Herzig said.

Prostitution and consumption of paid sex are legal in Switzerland. Prostitutes are considered self-employed and are liable for taxation.

"We are trying to promote the independence and self-determination of the sex workers," he said. "It's not a unique concept. We went to Utrecht in the Netherlands and Cologne and Essen in Germany, where similar projects exist, to speak to organizers. In Utrecht it's been successfully implemented since 1986."

As well as backing from the authorities, the project in Zurich has a democratic mandate: A referendum was held in March 2012, with 52% of voters in favor.