The French theater action group X TNT did an action they call Stress Free - Keep Walking - No running. Basically it was a flashmob creating a pedestrian crossing at the la Place de L'Etoile (also known as the Place Charles de Gaulle) in Paris.
That is the site of a huge, traffic-heavy and potentially dangerous roundabout for pedestrians in the center of the city. It's also a big intersection spot for 12 major avenues in Paris, and historically significant, too, as the location of the Arc de Triomphe.
Baron Haussmann designed the intersection of the different avenues at the Arc as a way for Paris to achieve monumental grandeur and greatness - he just hadn't planned for the automobile to take over urban space to such a great degree.
In fact, there's no longer any pedestrian access to the Arc de Triomphe from any of the 12 avenues, and so tourists and other visitors who want to get up close to this monument have only one choice - via a tunnel underpass.
By walking out into the traffic with giant white poster-like boards attached to two sides of their bodies, the 'actors' in X TNT's action basically stop the the heavy flow of traffic. Then they drop their boards, and voila! Instant Crosswalk.