A. Does the state have rightful authority in this matter? - sure, in our current system of publicly owned roads it does.
B. Fine, so the state itself is by and large* the measure of right/wrong in this case. Therefore, what does the state itself believe on this matter. The actual actions of state officials (police) become relevant:
a. it is common practise not to hand out speeding tickets for speeding < 15km over - almost no one takes the strict literal interpretation that going 1km over is a big deal - the actual line is subject to an officer’s judgement call
b. It is commonly understood that going with the flow of traffic is an important consideration, given that the primary purpose of the law is to ensure safety - this fact is commonly accepted by many police officers.
c. Given that no lives are endangered, there is no basis in natural law, or Church teaching for a given speed. The Church simply says follow the law of the land. A primary component of any law is enforcement. Given that minor speeding is almost never enforced, you can reasonably conclude that the state takes no serious issue with minor speeding violations.
d. as per safety, I would not be surprised if a buffer were built into the considerations for any given speed limit (ie: say the actual safe maximum in good conditions was determined to be 120km, and the buffer were ~ 20%, the speed limit would be set at 100, with the understanding that people would drive well under 120 under fear of being penalized.)
C. Granting that a given case of speeding is immoral, it is would definitely only be venial, unless there were some component of endangering life, in which case it could become grave matter. (I don’t think you specifically opined that speeding a given amount over was grave matter, but some have)
D. Reckless driving is a separate question, and may, or may not involve speeding. (Ex: cruising 30 over on a clear road with no traffic is much less dangerous than tailgating and driving erratically at 10 under in a blizzard. Arguably, the first is more likely to get you a ticket though…). Reckless driving could rise to the level of grave matter, while simply defying the state is a such a small matter would not.
My personal conclusion: Speeding in itself is not immoral unless you are endangering lives. However, the state reserves the right to penalize you, at which case you will have the moral duty to comply unless you are successfully able to challenge it in court. By speeding, you accept this risk.
*There are other considerations, such as the duty to not endanger life, which exists completely separately from any legislation
