But it does not save on signs - there was a very significant cost to alter existing 30 MPH signs from 30 / 40 to 20 and back to 40 and then to 30 and back to national. Last time I drove in Wales it was a shambles with speed limits changing every couple of hundred yards. Best bit was I was in a 20 then raised to 30 as it passed a school.
Don't get me wrong 20 past schools vulnerable people Care homes etc with ridged enforcement based on risk great. Blanket 20 mph based on well dogma is just pants.