Prior to 1998, a strict building height restriction was imposed in Kowloon City and over much of Kowloon to minimise the hazards of air traffic commuting through the Kai Tak Airport. The closure of Kai Tak as a result of the opening of the new Chek Lap Kok Airport lifted the height restriction, and more high-rise apartments started to appear, with heights of up to 175m. In more recent years, rapid gentrification has taken place and new residential skyscrapers have replaced old tong laus. However, the outline zoning plan described some of these recent high-rise developments as undesirable, and proposed a height limit of 80m for new buildings in Kowloon City.