So, who causes traffic congestion? The slow(er) bicyclist, or the car?

What you’re looking at is the visual representation of the difference between 35 people in single-occupant cars and those same 35 people in a multi-modal distribution. The municipal government of Denver, CO was the creative force behind this visualization.
Edit: Thanks to Jacquie Phelan for noticing that the old Denver photos (hotlinked) had been removed. I replaced it here with photos staged in Muenster to much the same effect.