Six European cities have been studied to determine the effects of introducing 30km/h speed limits on all or most roads within the cities. The results indicate that the lower speed limits have resulted in significantly fewer crashes, lower speeds, reduced noise, reduced local pollution, and no increase in travel time or congestion for vehicular traffic. Also, the reduction in crashes particularly related to serious crashes involving pedestrians and cyclists.

For more information, see:
https://www.toi.no/publications/introduction-of-30-km-h-as-general-speed-limit-in-european-cities-what-effects-can-be-documented-article38545-29.html