This article may not address this, but many articles of this type by Lenore Skazeny and others do address it. IIRC the findings:

- stranger danger was worse in the 70s than it is now. - safety in numbers was better in the 70s -- if all kids are outside it's more likely to be somebody else's kid that is snatched. If your kid is the only one, ... - car danger was worse in the 70s. Cars are bigger/faster now, but there were more drunk drivers then. This varies widely by jurisdiction.

It's hard to balance the factors -- it's not clear whether or not it was safer to let your kids outside today than it was in the 70's.