If you are interested in visualizing city traffic, you can of course use some sort of fancy camera tracking software and then create some crazy artwork out of the resulting video patterns. If you are not into programming, you can now use a cheap alternative: the project Farbenmeer shows how (trashing opulent quantities of) paint (in front of unsuspecting cars) can accomplish practically the same. Different colors were used for the different directions of a busy car crossing, so that traffic patterns become apparent.
And yes, the paint is environment-friendly and water-based. It took only some rain for the paint to disappear.