I think many people expect, when they buy a home on a quiet street, that the street will remain quiet indefinitely.
Obviously that's not an entirely rational expectation, but it's very understandable.
I grew up in an entirely enclosed (not gated, just not connected) neighborhood, where riding my bike as a young child was very safe. After I moved out, it was connected to new neighborhoods, so there is now non-local traffic. It's a fairly dramatic change for people still living there.
Yes. When was growing up from a relatively young age I could be gone half the day on my bike, traveling to friends' houses or nearby parks. I'm not sure I will let my children do that today because of the traffic through our current neighborhood.