The long-planned, $4.45 billion Second Avenue subway project is 82% complete and on track for its first phase to open by December 2016, Metropolitan Transportation Authority officials said on Thursday.

The first phase, which will run from the existing station at East 63rd Street and Lexington Avenue to a new stop at 96th Street and Second Avenue, is expected to serve about 200,000 riders daily. The MTA hopes that it will relieve congestion on the crowded Lexington Avenue line, which carries about 1.6 million passengers a day.

Michael Horodniceanu, president of capital construction for the MTA, said during a tour on Thursday that he was 75% confident the project will open by December 2016.

But some of the most critical work still needs to be finished, including installing communication, electrical, fire-safety and signal systems.

“These are the toughest 18%” remaining for phase one, Mr. Horodniceanu said. “It’s not just concrete, mortar and bricks.”