Around 25-30 sex workers are now using the guarded drive-in brothel, up from around 15 this time last year.
That's nearly the same number as the estimated 32 women that had been plying their trade on the Sihlquai, a street in Zurich's red-light district, before prostitution was banned there in August 2013.
Set up two years ago as a way of drawing prostitutes away from the Sihlquai, the wooden drive-in ‘sex boxes' can accommodate 50 prostitutes and are one of three spaces where they can work legally in Zurich.
The boxes are under 24-hour surveillance and feature facilities including a laundry, shower and cafe, alarm buttons and access to social workers and counsellors from the Flora Dora support network.
Plain-clothes policemen are regularly on site, while uniformed officers patrol the area.
In exchange, prostitutes working there must get a permit and pay tax.
Social services in the city yesterday feted the success of the scheme in protecting sex workers and improving their working conditions, reported news agency ATS.
No serious crimes have been committed there since the drive-through opened, according to police.
However sex boxes have not completely replaced prostitution on the city's streets, a police spokesman told ATS.
Police have stepped up checks in certain streets where prostitution is banned, he said.
Neither has the scheme kept within budget, with costs escalating to 50 percent more than originally planned.
However, the overall success of the project has made other Swiss cities take note.
In November, a politician from Basel proposed the establishment of sex boxes in the north-western city to eliminate problems in its red-light district.
Prostitution is legal in Switzerland but it is controlled by regulations and sex trade workers are required to have valid working permits.