It can also be used when you exceed the posted limit. While there is 22348 which applies directly to the posted speed, it only applies to the posting of maximum speeds of 55 or more on certain highways. If you do 40 in a 25 zone, you're going to get a 22350 citation. As I stated in the first response, the posted speed limit is the prima facie indication of an unsafe speed (though the code permits a defendant to argue that exceeding that was not unsafe by competent evidence for the situation involved.