The speed limit is the speed you should be driving in most circumstances, and is set far below the maximum safe speed for the road. Driving significantly below the pasted speed is as dangerous driving significantly above it.
Some roads have ridiculously low posted limits. Everyone goes well over, yet those roads are not noted for accidents. If they set the speed limit according to how everyone drives there, then I'd be in more in favor of alerting those who go even 1 km/h over that.
Driving significantly below the pasted speed Without good reason will get you failed in the UK driving test for not making adequate progress.
As my office colleague found out recently.
