Ah but that is a whole different issue, the problem is that for the last few decades residential areas have been designed more and more like high-speed areas, and that had a larger impact on driver speed than the actual posted sign. I cannot seem to find the original articles but here are a couple of related ones:

- https://www.citylab.com/design/2015/11/some-20-mph-streets-a...

- http://plannersweb.com/2013/09/wide-neighborhood-street-part...