We've heard about the big idea - taxing cars by weight and mileage, not engine type. We've looked at the science, the money, and the practicalities? But what about the politics? What could persuade governments to revolutionise the vehicle taxation system? How could this concept overcome the scepticism of the press, public, and politicians who don't like meddling in the costs of driving.
Those are the big questions we explore in this final episode of series 2.