I heard that in Texas if there was not an engineering study to determine the correct speed limit that the posted speed limit was not enforceable. I subpoenad an engineering study for a speeding ticket in Austin (all I received was a letter stating there was no study) but I didn't get to try the technique because the prosector decided not to go to trial. This was after he put me at the defendants table and brought in potential jurors
I kind of doubt it would have worked. Any lawyers out there know for sure?