In March 2016, Christchurch City introduced a 30 km/h zone within the central city. Two years on, here's an updated analysis by Glen of safety impacts of this lower speed zone, using the surrounding remainder of the CBD as a "control" comparison. Reported injuries: down 36% vs previous two years. Reported injury crashes: down 25%. All while vehicle and foot traffic have been growing with businesses returning to city.
(NB: a copy of the underlying analysis data and charts can be found here)
More recently, ViaStrada was commissioned to capture this analysis in a report for Christchurch City. A 2.5 year before/after evaluation found that the expected number of injuries (based on the changes in traffic volumes) fell by 33%, with a corresponding 22% reduction in expected injury crashes.
This is no real surprise, as the safety benefits of lower speed zones have been observed and researched worldwide for a long time. Some of this evidence formed part of the rationale used by ViaStrada when we undertook an investigation into a potential central city slow core for Christchurch way back in 2010 (even before the quakes forced a major review of transport in the city).