Complex sentence: The congestion was not cleared until I-95 was completed through Baltimore when the eight-lane Fort McHenry Tunnel opened in November 1985, which allowed the Baltimore Harbor Tunnel to be partially shut down for extensive maintenance in the late 1980s.
Split into simpler sentences: The congestion was not resolved until I-95 through Baltimore was completed when the eight-lane Fort McHenry Tunnel opened in November 1985. The transfer of most traffic to the new tunnel allowed the Baltimore Harbor Tunnel to be partially shut down for extensive maintenance in the late 1980s.