Reducing the speed limit in a 50 km/h area to 30 km/h results in considerable safety gains. The greater the decrease in actual driving speed, the greater the safety impact.
Many of the effect studies date back to the last century, when 30 km/h zones were introduced on somewhat larger scale. In the Netherlands Vis & Kaal  analysed 150 30 km/h zones without through traffic and with sufficient speed-reduction measures. They saw an average decrease in the number of injury crashes of 22%. However, there were large differences in effect between zones. These were mainly related with differences in zone size, degree of urbanization, nature of the chosen speed-reduction measures and the changes in traffic volume that were realized. Weijermars & Van Schagen  estimated that in the ten years between 1998 and 2008 the construction of 30 km/h zones saved a total of 51 to 77 road deaths.
Other countries than the Netherlands also report positive experiences with the introduction of 30 km/h zones, even though the reported effects differ. A meta-analysis of the results of 15 studies from the 1970s and 1980s by mainly Scandinavia, but also by Great Britain and Australia, showed a reduction in the number of injury crashes of 24% . A more recent before-and-after study of the effects of 78 30 km/h (20mph) zones in and around London, Great Britain  showed a reduction of 42% in injury crashes and a reduction of 53% crashes with fatalities or serious road injuries. A Swiss before-and-after study  reports an average decline in crashes of almost 15% and an average decrease in casualties of 27.5% after the introduction of 30 km/h zones at 21 locations in different parts of Switzerland.