http://oddlyspecific.failblog.org/2010/10/04/funny-signs-i-imagine-this-is-pretty-effective/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+OddlySpecific+%28Oddly+Specific%29&siteId=1000001&feedItemId=136982

Assuming they can read, a jump in some cases, I think this sort of thing might increase compliance.