Some road construction sites use smiley signs to influence driver behaviour: At the start of the road works the smiley is sad – but it gets progressively happier as you’re nearing the end of the construction zone. The goal is to keep drivers alert and decrease their frustration while they have to keep to a lower speed limit. In a similar vein speed boards in the UK have had a smiley added underneath. They smile at cars under the speed limit, and frown at those that go above it.

Actually a very good concept, I wouldn't have guessed this would work.