Milford, Conn.-based Subway plans to add between 800 and 1,000 more U.S. locations to its portfolio in 2011. In 2010, the sandwich giant opened approximately 1,000 new restaurants across the nation. Over the next five years, the restaurant chain hopes to open a significant number of stores in low-density markets, such as Philadelphia, Boston, Southern California and the outer boroughs of New York. The new restaurants will average between 1,200 sq. ft. and 1,800 sq. ft. in size. It is unclear exactly where the new stores will be located and when they will open. Currently, Subway operates about 33,966 restaurants in 95 countries worldwide.