As others have pointed out -
just a speed limit reduction will have limited utility, but worth talking about. The key is to redesign streets where excessive speed and safety are a big issue. Collectors are dangerous because they are designed to feel like you should be driving 60 or 70. People respond to the environment presented to them - and narrowing of collectors, more traffic calming measures will have a more lasting impact on public safety.
I tweeted this comparison of two Calgary residential streets a while back. One promotes speed, one promotes slowing down and proceeding with caution - automatically.
https://twitter.com/joshyyc/status/994973828341100545