grace area?

can you tell me what "grace area" means as well as explain what this whole sentence means? I'm confused by "combined difference between radar and speedometer error' part...

"a grace area exists of about 5 to 7 mph above the posted speed limit because of the combined difference between radar and speedometer error."

thanks in advance!

[Tdol]

It means that if you're measured as travelling slightly over the speed limit, you won't be punished because it could be accidental, caused by error in your speedometer or the radar. If the radar is slightly inaccurate and your speedometer too, then this could combine to make a larger error which you're not responsible for. Your speedometer could say your travelling at 70mph when you're actually travelling at 72mph, and the radar might say you're traveeling at 74mph.