Government action to reduce negative externalities

 ......in this case, injury and death! The spread of 20mph speed limits in London https://tfl.maps.arcgis.com/apps/instant/basic/index.html?appid=23ad4edbd905493089048e1bb3ea8da1

Do they work? The eveidence is mounting that they do https://www.google.com/search?q=have+injuries+and+deaths+declined+as+20mph+zones+have+increased%3F&oq=have+injuries+and+deaths+declined+as+20mph+zones+have+increased%3F&gs_lcrp=EgZjaHJvbWUyBggAEEUYOdIBCTM2NjA2ajBqOagCBrACAfEFdY-bZ4IUGCA&sourceid=chrome&source=chrome.ob&ie=UTF-8