The only goal is to get in and out of Boston in a reasonable amount of time. I wasn't around for pre Big Dig Boston but it's still dangerous and time-consuming driving to get out of Boston by car. The Big Dig might've made it _less_ dangerous and time-consuming, but the point is the solution barely scales since the total number of people driving just increased instead. If they spent those 20 years and billions of dollars on burying and expanding the T lines, and improving the commuter rail offerings, I wager we'd have achieved a lot more towards the aforementioned goal of getting in and out of Boston quickly.