As I said, it was one speed bump per mile average, though that number involved several any time coming to an important intersection or entrance to a town or business area. They put them any place where they really need people to slow down.
For example, there are some major chicken ranches half an hour from my house. So, they need agricultural inspection stations (Newcastle, etc) on both sides of each ranch. Each inspection station will have two speed bumps, one on each side.
Due to speed bumps and other things, it is hard to average over 25 miles per hour except on high speed toll ways. And, that 52 miles I mentioned, the speed limit over most if was 60 kph, around 38 mph.
High speed tollways do not normally have speed bumps. A couple places they put vibrators, like little round things sticking up on major down hill slopes. But, not speed bumps on the tollways otherwise. Well, they put something at the toll booths, I forgot that.
The rule of thumb is they put them wherever they really need people to slow down. Many years ago, a cousin told me they tend to put them where a pedestrian has been killed.
He showed me one in a small town not far from Cordoba. That sucker was more like a small mountain than a speed bump. He said four kids were killed by a driver, so they built an extremely large thing. Maybe 18 or 24 inches high and maybe 8 feet long. I did not have my camera with me, alas.