The “big dig” reduced carbon dioxide (co2) emissions by

I believe the answer is: 12%

The big dig was the most expensive tunnel project that ever existed in united states, which rerouted the Central Artery of Interstate 93, the chief highway through the heart of the city, into Thomas P. O'Neill Jr. Tunnel.

The reroute provide the citizens with additional options to reach these area without using personal vehicles, which is predicted to reduce the carbon emission for around 12%/