Reducing the speed limit in a 50km/h area to 30k m/h will considerably benefit road safety. The more actual driving speed drops, the greater the safety effect. A study in the early nineties , when the 30km/h zones were introduced at a wider scale in the Netherlands, an average 22% reduction of the number of injury crashes was found. In all, a further roll out of 30km/h zones in the first ten years of Sustainable Safety (1998-2008) resulted in an estimated 51 to 77 fewer road deaths .
These kinds of safety effects of a standard speed limit of 30 km/h will only be achieved if it is impossible to exceed this limit in practice. Merely putting up 30km/h signs will not suffice, since a 30km/h limit also needs to be credible (see the question What are credible speed limits?). Introducing a standard 30 km/h will be a challenge for through-roads in particular. It still needs to be determined how these roads can be redesigned as 30km/h roads so that they are safe and will also facilitate traffic flow to a sufficient degree.