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

• Early proposals
Albert Lilienberg was appointed city-planning superintendent in 1927, and a year later he produced the first proposal for a public square on the location in his general plan of 1928. In his proposal he envisioned a square whose north-south oriented axis would line-up with Sveavägen intended to be extended south across the square down to the waterfront with widened Hamngatan and Klarabergsgatan joining in from west and east. After this first proposal, the square is featured on every subsequent city plan produced for the area, with alternations in width and length. Notwithstanding the considerable number of revised proposals produced, surprisingly few were preoccupied by architectonic considerations, instead focusing on optimization of traffic flow.
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.