If you were traveling a speed faster than what a properly posted sign indicated, then that is Prima Facie evidence that you were indeed NOT traveling at a reasonable and prudent speed. The only way you could prove that you were traveling at a speed that was reasonable and prudent would be to prove that the actual legal speed limit was somehow different from the posted limit. My guess is it ain't gonna happen.
The Law here states:
"
§ 545.352. PRIMA FACIE SPEED LIMITS. (a) A speed in
excess of the limits established by Subsection (b) or under another
provision of this subchapter is prima facie evidence that the speed
is not reasonable and prudent and that the speed is unlawful.
Text of subsec. (b) as amended by Acts 1999, 76th Leg., ch. 663,
§ 2 and Acts 1999, 76th Leg., ch. 739, § 1
(b) Unless a special hazard exists that requires a slower
speed for compliance with Section 545.351(b), the
following speeds
are lawful: (1) 30 miles per hour in an urban district on a street
other than an alley and 15 miles per hour in an alley;
(2)
70 miles per hour in daytime and 65 miles per hour
in nighttime if the vehicle is a passenger car, motorcycle,
passenger car or light truck towing a trailer bearing a vessel, as
defined by Section 31.003, Parks and Wildlife Code, that is less
than 26 feet in length, passenger car or light truck towing a
trailer or semitrailer used primarily to transport a motorcycle, or
passenger car or light truck towing a trailer or semitrailer
designed and used primarily to transport dogs or livestock,
on a
highway numbered by this state or the United States outside an urban
district, including a farm-to-market or ranch-to-market road;"
The Poster above posted the law already. It is their if you want to read it. To me there is a case if this happened in a rural area. The OP even quoted the correct law to read. That is just my two cents.