We have shown the very clever billboards promoting water conservation in Denver before; They were part of a campaign that included benches, cars and just about anything Sukle Advertising and Design could come up with.
Paul over at frugal living site WiseBread thinks that it is a good allegory for life, not just water.
A phrase like "use only what you need" should be good advice for all of us, ingrained from a small age until the day we die. If we all follow that advice, think of the savings we'd make as a society; savings that could be passed on to us, the customer. So give a thought to anything and everything you consume over the next few weeks, from a simple printout at work to a whole meal for your family. Use only what you need; take only what you need; buy only what you need. Nothing more.
We are not crazy about the medium but love the message: More Billboards on TreeHugger
Less is More on Water Conservation Billboard
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Message Trumps Medium: Five Sort Of Green Billboards