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Many cities around the world want to reduce the negative impacts of extensive car traffic. One step in this direction is the reduction of its maximum speed limit.

This writing centers around three parts: First, I analyzed the official motivations and some experiences and drawbacks of fourteen cities – all of them European – which have reduced or plan to reduce the general speed limit to 30 km/h (20 mph). Second, I reviewed the scientific literature on the effects of reduced urban speed limits. Since these were almost exclusively limited to traffic-related aspects and ignored the wider social, psychological, and public space related aspects, I extended the review to studies which investigated the effect of (reduced) motorized traffic on these wider aspects, e.g. during street experiments. Third, I developed a survey that can be used to study these wider aspects. The survey was tested on different street types (residential, touristic/commercial, through street) with different speed limits (30 and 50 km/h). The first result is that the most-stated motivations for a reduced speed limit are reducing injuries/deaths, reducing noise, promoting cycling and walking, and increasing livability. Second, apart from traffic safety and noise pollution, the scientific knowledge about the effects of a reduced speed limit is scarce until now. In order to advance this knowledge, methods and tools should be considered that have been used to study the effects of (reduced) motorized traffic on social, psychological, health, and public space related aspects. The third result is, however, that these aspects are affected by numerous variables apart from speed limit, e.g. the width of the street and the presence of greenery or car parking. In addition, the wider effects of a reduced speed limit might evolve over longer time scales which could make them challenging tostudy in the presence of general trends or measures that reduce car traffic.