Texas and California are actually what are called Presumed Speeding states, unlike most others which are Absolute Speeding states. (There is a little known third category called Basic, but this is uncommon). In a presumed speeding state, a speed-limit violation offers someone in your shoes far more flexibility in building your defense than the more common absolute state.
In states that use this presumed system, such as California and Texas, it is not illegal to drive over the posted limit as long as you are driving safely and this can be established. For example, if you are driving 50 mph in a 40-mph zone, you are "presumed" to be speeding, yes. However, despite this prima facie evidence (meaning "on its face") of speed in excess of the posted limit, if you can show you were driving safely you may be able to mount a pretty decent defense. Just because you got a ticket is not prima facie evidence beyond a reasonable doubt that you were speeding. You are presumed innocent. If they prove you were speeding this is all they need to make their case, unless you rebut it. But there is a lot of room to rebut this presumption – and that is if they prove it. They must (as with all criminal cases) prove you did what you are accused of beyond a reasonable doubt – the highest standard of proof in the U.S.
So, if everyone was going 80 MPH in a 70MPH zone, you would argue that the road was (I'm assuming) dry, unmarred, you were traveling in heavy enough traffic that to slow your speed to the posted limit would actually be less safe than traveling with the flow of traffic. You can talk about the state of your vehicle (repair, handling, etc.), how you stayed in one lane, and you can describe your skill at driving – especially if you have no other tickets and you have been driving for a long time.
If you were weaving in and out of traffic, riding someone's bumper, if it was pouring rain, the road was bumpy or under construction, or if you admitted speeding (if you did this you can still rebut with a showing of safe driving, but ignore all info regarding challenging radar or other means of determining speed), or if any other evidence exists that the officer would testify to that shows you were not driving safely, this will not be successful. A successful example of a speeding defense in Texas: on a clear, dry morning with no other cars on a wide, straight road, a man is pulled over for being clocked going 50 mph in a 40 MPH zone. He had a perfect record and had been driving 22 years. He was driving a 2-year-old car. He convinced a judge that this was driving safely given those conditions and was acquitted. That's because facts presented were sufficient to "rebut the presumption" that by going over the posted limit he was driving at an unsafe speed.
NOTE: Never bring up your driving record unless it is spotless. Unless you are a habitual offender it cannot be used against you or be brought into evidence at all, unless you open the door.
You can also mount a defense based on the radar detection device, if one was used. You can seek records as to when the calibration fork was last checked, when the last time it was professionally calibrated (rather than self calibrated). You can ask, in a leading way (only if you know) how close the car behind and in front of you were (you don't want to be too close to the car in front of you, however if the cars were tightly grouped it is more likely the radar detector could have read another vehicle): e.g., "Isn't it true that the car behind me was only 1.5 car lengths behind me?" Only do this if you know, but if you can get the officer to admit that the car behind you was close, that can be used to rebut the radar detection and goes to the argument that driving slower would have been dangerous and you were driving safely with the flow of traffic.
You should be prepared to put on an entire trial if you fight the ticket. In Texas, I believe speeding is considered a Class C criminal offense (rather than a civil offense as in most absolute states); hence, they have to build the prima facie case against you and prove it beyond reasonable doubt. If you can afford one, get a good traffic violation attorney. Always choose a jury in this type of case. Everyone speeds a little and you are far more likely to be acquitted by a jury than a judge. You should also ask to have the case assigned to the county seat; request this in writing ASAP.
If you are trying the case, be prepared to go after the officer. Note any distinguishing marks on your car (if any), recall what you wore, what time of day, the lighting, all that. Even go back to the scene at the same day and time and take video showing the flow of traffic, (hopefully) the straightness of the road, etc. Cross-examine him on all facts with confidence and in a leading manner. Always ask for the calibration reports and you will get all evidence against you in discovery.
This thing about 10mph being the minimum they can give a ticket for: ignore that, it's rubbish! It's meant to get you to admit to him that "you were only going 8 or 9 over." Also, that whole percentile argument is not relevant and will not work at all. You must show that you were driving safely given all the facts and circumstances to rebut the presumption that you were driving unsafely by speeding.
It is worth fighting as you will also incur surcharges, increased insurance rates, and points on your license that are cumulative and stay for 3 years – a certain amount of which gets you suspended if you get (or have) more violations.
