Here are some maps highlighting a wide variation in the speed limits in force in urban areas in different towns and cities based on OpenStreetMap viewed using the Speed limit mapping view of ITO Map. Oxford and Berlin both make considerable use of 20 mph limits or the km/h equivalent (shown in green on these maps) with most of the other roads at 30 mph (shown in orange on the map). Norwich and my home town of Ipswich make some use of 20 mph but use 30 mph for most residential roads and have some core roads operating at 40 mph limits (shown in red). Stevenage has much higher roads with a core network operating at 40 mph and nearly all of the rest operating at 30 mph. This may be due to Stevenage being a ‘new town’ and having been built with separation of pedestrians and motorists in mind from the start. In Milton Keynes the core network operates at 60/70 mph and pedestrians only use these roads to access bus services. There is limited information in OpenStreetMap about the speed limits for most of the rest of the residential roads in the city.
It is worth remembering that a child hit at 40 mph has a 90% chance of being killed, at 30 mph it is 50% and at 20 mph it is 10%. I will publish some maps in due course showing both speed limits and road casualties. In the mean time why not help improve the coverage of speed limits in OpenStreetMap?