Since then, Boulder has adopted a policy of controlled urban expansion. In 1959, city voters approved the "Blue Line" city-charter amendment which restricted city water service to altitudes below , in an effort to protect the mountain backdrop from development. In 1967, city voters approved a dedicated sales tax for the acquisition of open space in an effort to contain urban sprawl. In 1970, Boulder created a "comprehensive plan" that would dictate future zoning, transportation, and urban planning decisions. Hoping to preserve residents' views of the mountains, in 1972, the city enacted an ordinance limiting the height of newly constructed buildings. A Historic-Preservation Code was passed in 1974, and a "residential-growth management" ordinance (the Danish Plan) in 1976.