Hold up there, Tex. The law specifies that driving over the posted speed limit is prima facie that you are driving in a manner that is not reasonable and prudent. This allows the cop to ticket you [Texas Transportation Code Sec 545.351-2]. If you went to trial for a speeding ticket it is possible that you could argue the common speed of vehicles under similar conditions was what you were driving, and you could MAYBE get out of the ticket that way (although speeding about 70 in most counties, 80 in some is likely an absolute that you couldn't get out of), but you'd be relying on the whim of a judge or jury in that case.
The Texas Administrative Code specifies the procedures for setting speed zones [Title 43, Part 1, Ch 25, SubCh B] specifies how speed zones are set. IANAL, but I found nothing that says, "if people speed, you have to change the speed limit." It does seem to imply that, at the discretion of certain regulatory bodies and after an engineering study, a speed limit can be raised.
I'm also a lazy Texan, but Google works for me.