This is going to be fun. Some people will learn that it's soft at slow speeds and will assume it's always soft and severely damage their cars.
It's typically done in "problems areas" where a small number of people drive 25-50% or more over the limit, but it's extremely frustrating, because it forces drivers to drive way below the actual limit when crossing the speed bumps - especially if they're the bolt-down kind. i.e. the effect is to cause irritation for ~99% of drivers in order to punish a tiny minority.
[1] I'm sure it happens elsewhere in the US, too.