I have actually seen some cities where they post what speed you should drive at to optimally get green lights.
Now that cars are getting more hi-tech, it'd be a great thing to add to the dashboard.
"You're going Xmph but you should be going Ymph to save gas."
I have seen this in a rental car sometime before. But its displayed in a different way. It shows how many miles you get per gallon when you drive at the current acceleration. Just a number that changes when you press or release the accelerator.