@ossobuco,
According to the article -
The protest was intended “to confront white complacency in the systemic oppression of black people in Boston,” the Boston contingent of Black Lives Matter said in a statement.
“Today, our nonviolent direct action is meant to expose the reality that Boston is a city where white commuters and students use the city and leave, while black and brown communities are targeted by police, exploited, and displaced,” protester Katie Seitz said in a statement.
So though they don't say it is to stop traffic directly - say they are exposing commuters lead me to believe that the intention is too stop traffic.