What's the point of having laws, if your citizens can't possibly know/remember them?
Which leads me to believe that the speed limit is only there so the police have a reason to pull over anyone without any other reason. Don't like the look of the driver? Pull them over, while ignoring every other car doing 70.
Who knows if it would get thrown out in court, but most people plead guilty to traffic citations by just paying the fine anyway, and the system knows that.