Boston officials say they’ve gotten state and federal approvals to install bike lanes on Massachusetts Avenue, a key downtown artery, from Boston Medical Center to Symphony Hall. The lanes will be added as part of an $18 million reconstruction of the avenue, the mayor’s office said The project will be 80 percent federally-funded and 20 percent state-funded. Plans call for it to be completed by fall 2011. “This is a big win for Boston’s biking community,” Mayor Thomas M. Menino said in a statement. “Massachusetts Avenue is one of the busiest and most heavily traveled streets in our city and this project provides an opportunity to accommodate all modes of transportation without compromising safety.” Menino announced the new bike lanes at the Boston Bike Festival, the final event of the first statewide bike week. People today joined convoys of bikers in Boston and various close-in suburbs and were guided in convoys into the downtown, where they were treated to the festival on City Hall Plaza, which included a free breakfast and various other freebies.