If you have a speeding ticket in Texas, how exactly could a Speed Zone Study help to defend against the citation?

If the ticket says, for example, 80 mph in a 70 on an interstate (which is the lowest possible citation in a 70 zone, according to an officer), what information from a traffic study would be sufficient to overrule such citation in Texas? (And assume that the flow of traffic already contradicted the posted limit at the time of the citation.)

Obviously, if the study were to say 80 mph, it'd be all clear, but it's more statistically likely that a study would find an 85th percentile speed to be some number between 72 and 77 mph, then what?

1 Answer 1


You seem to misunderstand what prima facie means: accepted as correct until proved otherwise.

The facts are that you exceeded the posted speed limit; therefore it is legally accepted that you were driving at a speed "greater than is reasonable and prudent under the circumstances then existing".

To overturn the prima facie assumption you must prove beyond reasonable doubt (for a criminal matter) or on the balance of probabilities (for a civil matter) that the speed at which you were travelling was "reasonable and prudent under the circumstances then existing". Demonstrating that other people were also driving at this speed does not show that you were "reasonable and prudent" because prima facie they were themselves not "reasonable and prudent".

A possible (if unlikely to succeed) defence would be to demonstrate that the combination of your driving skill, your vehicle's performance and the road conditions were such that you were as or more capable of avoiding a collision at 80mph as the general population is at 70mph. A professional race car driver in a high performance vehicle on a perfect driving day might have a chance with this.

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    You sure it's beyond reasonable doubt? Speeding in the US is often civil; plus, I'm not sure when anyone but a criminal prosecution has that standard. I'd expect at a maximum clear and convincing, and more likely preponderance of the evidence.
    – cpast
    Aug 27, 2015 at 2:01
  • Ok, but don't the states have to follow the law about how speed limits are set? Doesn't federal law require states to use the 85th percentile to set speed limits? If the speed limit was not set according to law, wouldn't this trump the prema facie law?
    – Andy
    Aug 30, 2015 at 20:17
  • @Andy, there are two ways that federal law can trump state law: either (a) the Constitution says it does, or (b) the federal government has tied funding to the state passing certain laws. I'm pretty sure that speed limits aren't mentioned anywhere in the Constitution, and if (b) applies, it doesn't make the state law invalid, it just means the state won't get the funding in the future.
    – Mark
    Aug 8, 2016 at 8:45
  • The OP says the state is Texas. In Texas speeding is criminal; a Class C misdemenour.
    – mark b
    Feb 3, 2017 at 1:41

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