Roads that are safer for pedestrians and cyclists get more people out of their cars. Longer car commute times make people consider alternatives such as public transit, walking, or biking. Every additional person who isn’t in their car has an exponential decrease in automobile congestion. This is all relatively well understood within urban planning and traffic engineering.
I don’t believe that was the same group, but I’m fairly certain that protest just involved leaving a note on the windshield and using a lentil to deflate the tires. Their notes did touch in the environmental impact of SUVs, but from what I recall the notes focused on the negative impacts that SUVs have in cities. Such as the skyrocketing number of pedestrian deaths we are seeing.