Ha! I was wrong. Comprehensively. On looking at the stuff pointed out by @UDS1, I see an actual offense of “loss of traction” , ( ROAD SAFETY ACT 1986 - SECT 65A), and exceptions proving the rule: the exception for private property applies if it’s a venue, or according to the rules of a motoring organisation.
That second exception looks broad enough to drive a truck through: I don’t see a definition of ‘motoring organisation’, but I’ve already demonstrated that IANAL.
In the state where I asked, the only enforceable speed limit signs are black-on-white and meet other legal criteria, such as size or height. Any other speed limit sign is not enforceable per se, but can be used if there is an accident, to show that speed contributed to reckless driving. (Kansas)
About reckless driving on parking lots. In Finland every parking lot is defined as a pedestrian zone eg. a zone where pedestrians can walk where ever. This is due to the fact that people must get in and out from the cars and walk around the lot. So cars must by definition yield to any pedestrian. This precludes any sort of recless driving on a parking lot in Finland.
I’ve been in a heavily forested area which had wooden (IIRC) speed limit signs, yellow on brown. The limit was 75 MPH and enforcement was handled by the many trees, anyone actually trying to drive that speed likely would have ended up wrapped around one.
I would imagine that that exception is for places like racetracks. Or a venue specifically set aside for people to do things like donuts (I don’t know what that would be called, but I expect that such venues exist).
In most states, there’s a default speed limit set by law for any road that doesn’t have any other limit posted. In Ohio, it’s 55. One of my aunts lives on a very hilly rural gravel road, where nobody ever bothered to put up a sign. The fastest I’ve ever traveled in a vehicle was one time I was with my uncle, going down that road at the speed limit.
a) I’ve seen more than one gated community with custom speed limit signs that have strange listings – 8 MPH, 16 MPH. 
b) Keep in mind that not all official looking traffic signs are legal. I got a ticket once pulling into a parking lot and it turns out the sign was not put up (or approved, apparently) by any government agency. The tic was nixed.
The strange numbers are an attempt to get you to pay more attention to the sign.
Or help you win a scavenger hunt on Family Guy.