There's no good reason to mandate this. It would just cost some countries a lot of money to change this. Just follow the rules of the road for the country that you are in. This is completely different from the idea of everyone switching to the metric system. As much as I'm used to the English system of measurements, I fully admit that the metric system is much more logical, and a lot easier to do calculations and conversions with. But there's no real difference with which side of the road you drive on.