Those roadside speed signs are surely accurate. I have 2 near my house and severl more i go by regularly and they all read exactly the same according to my speedo, so they are obviously calibrated meticulously to the same speed ragardless of what that is.
now if you think what that is is undercalibrated, how do you supposed they get away with that when lots of people have GPS and can test them. No way they would hand out tickets based on those. There would instantly be a big buzz about that and the police would have big problems and have to pay back potentially millions in tickets. they can't even get away with uncalibrated radar guns today, so having lying speed signs all over the place just ain't gonna happen. i assure you they are accurate. further proof is i have seen people say that according to thier GPS the Tbird's speedo is off by the exact same amount mine is, and that means all the speeds signs i see are spot on.