By the 1960s, Curitiba's population had increased to 430,000, and some residents feared that the growth in population threatened to drastically change the character of the city. In 1964, Mayor Ivo Arzua solicited proposals for urban design. Architect Jaime Lerner - who later became mayor - led a team from the Universidade Federal do Paraná that suggested strict controls on urban sprawl, a reduction of traffic in the downtown area, preservation of Curitiba's Historic Sector, and a convenient and affordable public transit system. This plan, known as the Curitiba Master Plan, was adopted in 1968. Lerner closed the XV de Novembro St. to vehicles, because it had very high pedestrian traffic. The plan had a new road design to minimise traffic: the Trinary Road System. This uses two one-way streets moving in opposite directions which surround a smaller, two-lane street where the express buses have their exclusive lane. Five of these roads form a star that converges to the city centre. Land farther from these roads is zoned for lower density developments, to reduce traffic away from the main roads. A number of areas subject to floods were condemned and became parks.