We'll see if the court will hear the case, I had a red-light ticket in Denton where you can see my car stopped behind the line, but my options were to pay $60 or go into arbitration with a company in Arizona. I know from my brief experience working with the City of Denton and general municipalities in OK that independant engineers determine a range for each light (and speed limits) and the local government always takes the value most likely to generate income. So I promise you that 3.1 is within the legally acceptable range.
For the record, the yellow light duration at State School Rd and in Denton is absolutely comical at under 2-seconds in length...much like the 30mph speed limit on Dallas drive which is a 6-lane road with a median at roughly 150' across.