Ultimately the limit is mostly decided by the width of the road. You can set a 10km/h limit but if your road is wide and straight, many will just go at whichever speed they feel comfortable with. The reverse is also true: I can bet it was very rare for anyone to ever reach 60km/h on streets like those shown in the picture.

So to really enforce slower traffic, narrow down your street to the bare minimum required for a car to pass through. Add obstacles as small turns or bumps. You'll also gain space for sidewalks and cycling paths.