The benefit of this is _less_ nannying in other areas. If the cars on your street do 20 mph instead of 35, you might be more likely to let your kids play outside.
What about my neighbors who might be child abductors or rapists? Or the animals, like a beer or eagle who might fly away with my kid? There's always something to be afraid of for people who are afraid.
IMO we would be better suited by building cities and spaces that are more "car free".