In the fourth study on littering Cialdini, Reno, and Kallgren found that when an area filled with litter was swept into piles or not swept at all, subjects:

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The study found that when there was already litter scattered around, subjects were more likely to litter. However, if the litter was swept into piles, subjects were less likely to litter, especially if a confederate also refrained from littering. This suggests that a clean and organized environment may deter littering behavior.

In the Fourth Study on Littering Cialdini, Reno, and Kallgren