It is the POSTED speed limit, not the actual speed limit.
Your speedometer is a liar. Car speedometers are generally 5% high.
At an indicated 50 mph, you are actually going just 45 mph.
Basic Speed Law --
http://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=VEH§ionNum=22350.
No person shall drive a vehicle upon a highway at a speed greater than is reasonable or prudent having due regard for weather, visibility, the traffic on, and the surface and width of, the highway, and in no event at a speed which endangers the safety of persons or property.
If you understand the Basic Speed Law, you can ignore the posted speed limits. I have ignored the posted speed limits for 50 years, never have got a ticket. If you don't understand the Basic Speed Limit, you should stop driving.