Surely the thing is that - after years of driving with a 30 mph limit in towns - people get a 'feel' for what that is like. That is, they know without too much speedometer-checking when they're going over it.
When, all of a sudden, that limit becomes a quite unfamiliar regular speed of 20 mph, drivers are obviously going to spend far more time looking down at the dashboard than usual. This will apply until that becomes the familiar speed-limit. If you're lookin' down, you ain't lookin' out!