I think in at least some instances of projects like this, the goal is not really "traffic reduction", even if that's what officials publicly claim to sell it to the public. As you say, the phenomenon of induced demand is well known & presumably the people involved in driving these projects are aware of it.
The goal is increased throughput/economic activity. If it takes X time to get a vehicle from A to B, and then you add lanes & demand goes up to keep X relatively constant, that can still be a net gain for community. It's about as equally frustrating for the folks sitting in traffic, but there's still more vehicles getting from A to B because there's more lanes, and more vehicles moving around can be good for the local economy.
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.
> The goal is increased throughput/economic activity.
They've still chosen an incredibly stupid solution if that is their goal. Building more highway is the least efficient means of improving transit throughput.
If all the roads are one lane, and most people can't drive their car every day, are they going to be buying more cars and trucks? I think having every road be narrow would reduce car and truck related economic activity.
The goal is increased throughput/economic activity. If it takes X time to get a vehicle from A to B, and then you add lanes & demand goes up to keep X relatively constant, that can still be a net gain for community. It's about as equally frustrating for the folks sitting in traffic, but there's still more vehicles getting from A to B because there's more lanes, and more vehicles moving around can be good for the local economy.