I had an idea. Some cars come with a feature that allows the user to see the current MPG (or KPL if you're across the pond). I think this hardware should be enhanced, so that when you put gas in your tank, you also put let the car know how much you just paid per gallon (or Liter). Rather than showing you how many miles you get per gallon, it would show you how much money per mile your current driving habits are costing you.
I bet people wouldn't slam the gas pedal down when the light turns green if they had something telling them that it was costing them $2.00/mile to drive like that.