I don't know about where you guys drive, but where I drive I feel that driving at 20 mph UNDER the posted speed limit on a major highway is equally as dangerous as driving over the limit.
Our highways are so packed that you would be seriously impeeding the flow of traffic, and waiting for a driver going 60 mph+ to crash into you from behind not expecting a car to be driving that slow.
It is actually a serious offence here to drive under the speed limit, they will take your car off the road and impound it and you get your licence suspended on the spot for 7 days. You also face charges between 2000-10000 dollars and a possible criminal record, and a very very long licence suspension.
This is the same fine you recieve for other dangerous acts such as street racing, stunting, and driving 50+ km/h over the speed limit.