The world’s top automakers are increasingly offering more electric vehicle models, as they steadily back away from gasoline engines. This growth is expected to continue, with more people choosing to hit the road with clean-running electric-powered cars. Environmentalists have praised the automakers for taking major steps to limit harmful pollutants linked to worldwide climate change. But electric vehicles are also known for not producing another kind of pollution: noise. They run on batteries instead of fuel, and can operate in silence. While many people might consider this a good thing, quiet cars can also cause problems. The main danger is that people walking in areas around electric vehicles face a greater risk of being hit if they cannot hear the cars coming. Governments in the United States and Europe have recognized this problem. So, they have set requirements for manufacturers to add warning sounds to vehicles operating on electricity.