Today's Boston can't even build a traffic tunnel. The Big Dig was a disaster-- took decades, costs billions more than planned, and was so shoddy the finished roof fell and killed someone. http://en.wikipedia.org/wiki/Big_Dig
As someone who works in an office building along rose kennedy greenway, I wouldn't say it was totally a disaster. Basically everything along the greenway has been revitalized and it's pretty beautiful to walk around in the summer. There is a lot more activity in the area and lots of new restaurants.
The real problem with the Big Dig is that is was designed and built in ignorance of the law of demand [1] as it applies to lane capacity. The traffic congestion that the Big Dig was supposed to eliminate simply moved outward [2].
They should have eliminated the Big Dig completely and spent the money on increasing rapid transit capacity.
That said, eliminating the elevated highway that cut the North End off from the rest of the city seems to have been a major boon.
I can't really speak to traffic because I rarely drive, but I do agree there needs to be more investment in rapid transit. I rely on the red line/walking to get to and from my office. It is the only line I would consider living on.
But I did want to add that it isn't only the north end that was cut off. I doubt the resurgence of the Sea port area/"innovation district" would have happened if it hadn't been for the big dig.
