Variable message (road) signs are designed and have been used to give warning / hazard notices to road users. In the Australian Capital Territory (ACT) they are used to tell road users about water conservation measures on a daily basis.
I have asked Doug McKenzie-Mohr if he has seen these signs used for affirmative messages. He says he believes not. I am asking via the forum to widen the net.
Christopher Yardley
Australian National University - Master student
Australia
Using Road Signs to Provide Community Feedback on Water Use
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Not sure if this is related to what you have in mind, but this is something that is catching on in my hometown of St. Paul, MN:
http://www.paintthepavement.org/
And also:
http://www.stpaul.gov/index.asp?NID=2820
Janna Caywood
United States