That's really not true in this case; this is not a situation where we're talking about local, intercity transportation; to suggest that traffic on the 1/4-mile-long Charles River Dam Road, which is used mainly to connect to major arteries in and out of the city, is going to get better by adding a bike lane is not rational.
I'm not saying traffic will get better. In fact, I'm even saying we shouldn't care whether vehicle traffic gets better—it's the wrong metric and has been for the last 60 years. We should care that more people can travel through the road.
Because, in fact, this is local intercity transportation. Getting from much of Cambridge to North Station is around 2 miles, but the bridge is horrifying to bike across and turns away potential riders. Same with the North End, or much of downtown—this is a direct route for biking, and making it safer will make it a better option for more people instead of taking an Uber, a convoluted public transit trip, or not making the trip at all.
There are many reasons to support bike infrastructure. We have a crisis of social isolation and sedentary behavior in this country, leading to unprecedented levels of metabolic disease and mental health conditions. That should be reason enough. If it's not, then understand that this area is, whether you like it or not, used by bikes, and it's incredibly unsafe; it is necessary to do something, now, to protect them. You don't have to buy into the bike revolution culture to accept this fact.
I agree, but I don't think there's actually a conflict between our arguments like you're implying.
The tones on this debate could, as usual, be moderated a bit. It doesn't have to be a war.
Someone who prioritizes driving in and out of the city over living in the city posted with caps and exclamation points, without an effort to imagine how much better this will be for residents of these actual cities.
And, if the city/people really care enough to sink the money in, they could build a cycle boardwalk over the water to connect Nashua Street Park to points north in the future.
We have the space to move people more efficiently and healthily already – we just need politicians to be willing to make the changes necessary. Kudos to MassDOT for their continued conversation with the bicycle and urban communities to make this street safer for all road users.