[Fat Charlie]

The US has trained its drivers that speed limits have no legitimacy and the signs are simply used to display a baseline number which may be above or below the actual limit. There is a complex formula that drivers need to apply to that number to figure out at what speed they are at risk for being stopped:

What kind of road are you on?
What time of day/week is it?
What the kind of car are you in?
What is its apparent state of repair?
How heavy/light is traffic?
How fast is this other traffic?
What color is your car?
What color is your skin?
How cute are you?
Is your license plate local or out of state?
Does your out of state plate bring to mind any sports rivalries?
What sort of bumper stickers do you have?

All these and more affect what the speed limit actually is and how you will be treated if you are stopped. The PSL is just a starting point for your calculations. Good luck out there.