I think the biggest and most important impact of this is safety actually. Survival chances from a 30km/h crash are significantly higher than at 50km/h. Crashes also simply happen much less because of a shorter stopping distance. Another thing is that at 30km/h you have much lower noise levels which is imo a public health issue that is very much undervalued. Also wear on the road surface decreases which makes for longer periods of time without construction.