In his book Unsafe at Any Speed: the Designed-in Dangers of the American Automobile, published in 1965, Ralph Nader derided the car manufacturing industry and US government for not harnessing available knowledge to ensure the “automobile was responsive to the safety requirements of motorists.”1 The crashworthiness of cars has come a long way since then, with most motorists in higher income countries surviving serious crashes. However, the problem of speeding has increased over time, placing pedestrians, cyclists, and other vulnerable road users at much greater risk of injury than the occupants of cars.2 Many road safety experts consider excessive and inappropriate speed to be “the single most important contributor to road fatalities around the world.”2

In the linked interrupted time series analysis (doi:10.1136/bmj.b4469), Grundy and colleagues quantify the effect of the introduction of 20 mph (32 km an hour) traffic speed zones on road collisions, injuries, and deaths in London.3 They linked geographically coded police data on road casualties in London to a detailed database of road segments and found …