Der har været forsøg med det i flere storbyer i Europa.

https://www.nrso.ntua.gr/review-of-city-wide-30-kmh-speed-limit-benefits-in-europe-may-2024/

Results from 40 different cities across Europe (including Paris, London, Brussels, and Helsinki) demonstrated that 30 km/h speed limits have led to significant reductions: 37% of road crash fatalities, 18% in emissions, 2.5dB in noise pollution and 7% in fuel consumption