There's also additional benefits from the fact that moving the highway underground made the area more desirable. There's no longer a massive concrete snake splitting downtown into pieces, and the area is cleaner and quieter due to moving all that car traffic underground.
In the specific case of the big dig, absolutely! I moved to Cambridge right around when the dig was completed and saw those improvements accrue over the next few years. I had a bunch of friends eventually move to Southie & The Waterfront, which would have been unimaginable when I moved there. I also think the dig did make navigation around Boston easier, even if it didn’t reduce overall traffic.
