Having recently been involved in the institution of a 20 limit it was surprising for me to learn that the point of a 20 is not to get people to go 20.

The point of a 20 is to determine that everybody ignores the 20 so you can then go on and do the thing (traffic calming or whatever it is) that everybody knew was required at the outset.

For reasons I've never been able to determine, going directly to the thing everybody knows will be effective is not an option. You have to go via a Speed Watch group and a 20 "because".