"Silent cops" were the hemispherical lumps right in the centre of a 4 way intersection. You were (In Oz we drive on the left) supposed to go across the intersection and turn hard right around the lump to make a right turn.

Then they changed the road rules to allow you to 'cut the corner' and do a 'diagonal' right turn. That's when they dug up all the 'silent cops'.

Robin