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The report contains an update and new analyses of the Power Model of the relationship between (changes in) speed and (changes in) road safety. The updated analysis is based on 115 studies containing a total of 526 estimates. The Power Model provides a good description of the relationship between speed and road safety. One version of the model has been developed for roads in urban areas, another version for rural roads and freeways. The effects of changes in speed on road safety are smaller in urban areas than in rural areas. The report analyses the normative foundations of speed limits. It is concluded that speed limits are needed, as a free choice of speed is unlikely to produce outcomes that are optimal from a societal perspective.