In a recent Critical Mass ride in San Paulo, Brazil, (AKA São Paulo) some of the riders had some interesting ideas in how to express their message and decorate their bicycles. One of them include signs (in Portuguese) that roughly translates into English as "1.5m the Law, 1 vehicle, 1 person"(see picture to the right. Click on any of the pictures to enlarge it.)
Another rider did a variation of that. I presume that either it was to playfully demonstrate that their bicycle is a car by giving it a boxed shape. It was referred to as a "bike car". I think there are two other ways of looking at this design. One is that this is how much space the bicyclist would be taking up if they were in a car, in this case a very small car. Another possibility, is that the perimeter of the box shape around their bicycle is the amount of clearance a bicycle needs to safely maneuver in the street. I believe that would specifically be referring to side clearances. In any case it was creatively hung to the bicycle. The poles in some were of the cardboard cyclinders. Some were of a cylinder form material. See more pictures and read the Google tranlated version from Portuguese to English.
I also like that they attached colorful balloons to the corners of their "bike cars". It adds to the festive nature of the ride, as well as drawing attention, and may even contribute to providing more visibility to their box or triangle shape pieces of art.