Mind Your Step Illusion By Erik Johansson, Sergel's Square, Stockholm, Sweden

In the city plan Helldén produced in 1946, the square, still named Sveaplatsen ("Svea Plaza"), was conceived as similar to the present square, but still remained an unarticulated modernistic concept. In this proposal, the square was centred on a rectangular open space furnished with trees, benches, and ponds; a space reached by subways stretching under the surrounding roundabout. During the 1950s, continuously increasing traffic loads made separating pedestrians and car traffic desirable, and several studies produced around 1955 focused on a lower level for pedestrians with cars on street-level with various openings to allow light down to the pedestrians.
• Final proposal
The hexagonal building ("Muttern"), a coffeehouse, was removed in 2005 in favor of a street level entrance to the metro station.
In 1957, a first official proposal presented a square virtually similar to the present; except that instead of the fountain there was an opening with tall trees and on the western side, where the flight of stairs is today, was a building was standing on pillars. The Chamber of Commerce was critical of the concept, concluding pedestrians on a lower level would produce poor business sites, an analysis which would eventually prove correct. Their own proposal the following year, developed together with various authorities, reserved street-level to pedestrians while cars were confined below ground. This counter-proposal was however produced in only two months, which made it easy for opponents to pin-down its weaknesses (mostly a failure to leave enough space for the metro which was being constructed at this time). Nevertheless, Helldén's proposal failed to impress the city as well, and Helldén together with other hand-picked experts was therefore sent on a tour around Europe, including Coventry and London, to find a better solution. In Stuttgart they could conclude that having pedestrians on a lower level required escalators, and in Vienna the pedestrians hall Opernpassage gave them the inspiration to replace the central open space at Sveaplatsen with a round restaurant with glass walls, an aesthetic device intended to give the square an architectonic dignity.