Not sure if the Big Dig is a great project to reference here, it went over budget by some 200% and many years. It was seen as one of the greatest civil engineering mistakes by the end. Sure it improved the QoL, but what other projects that take less than some 15 billion dollars can improve QoL just as much?

The Big Dig was one of the "cost-plus" projects mentioned in the article. The contract was for a guaranteed 7% profit on top of costs for design and construction. Is it any wonder that the construction company has zero incentive to do it efficiently or reduce costs! Overruns only bring benefits (more work, paid for by the state), and no downsides.

Another industry that loves their cost-plus projects is interestingly the traditional rocket launching business. Boeing and friends has for decades gotten used to massive cost-plus project contracts from the government for their launches, and is now feeling the pain of having to start to compete with Space X that operates at a magnitude or more efficiently and continues to drive down the price despite already being the cheapest.

There are lots of lessons to learn from the Big Dig - but I don't think one of them is that the eventual cost and time overruns were strictly necessary. Also: regardless of the implementation issues I think it is interesting to look at how it has affected Boston after completion. If only to identify any positive effects and see if you can work out how to achieve similar ones at lower cost.

You don't need a new tunnel and highway to fix neighbourhoods and improve walkability. There are plenty examples of cities that simply removed highways and didn't replace them with anything, instead relying on increased public transit options to continue to move people around the city.