2 Big 2 Fail
Contributor
As far as traffic stops, if I answer the "do you know why I pulled you over" question with "because I was speeding," that is an admission. It is admissible against me in a traffic trial. All of my protestations to the contrary will do me no good. Neither will an officer's inability to establish the radar unit was working properly or was aimed at the right car. However, without the confession, if the officer does not prove speeding, the case is over.
In reality, the only evidence that "works" in a speeding case is the speed measuring device, shown to be reliable, and proving up the speed limit.* You could not, for instance, pull up to an officer beside the road and say, "I was speeding back there. Write me up." and expect to be cited. (Okay. If he had a sense of humor, he might send you off with a ticket, but more likely you'd be detained for a bit, but not for speeding.) If the state can't make its fundamental case on it's own, your admission won't matter. Of course, if you happened to encounter an amazingly inept officer who couldn't establish his radar evidence, you lucked out, and again, your admission won't matter. But that's terribly rare. (Or shouldn't matter. Not all lowest level courts are very rigorous about the law, but if you find yourself in one of those, nothing about the radar or anything else will matter much.)
But speeding citations and the somewhat casual arguments in traffic court don't have much to do with the sort of case at hand.
* I also spent some years as a judge in such a court. I did find one guy not guilty of speeding in a bench trial, because it was one of those school zones where it's posted, "Speed Limit XX When Light Flashing." The officer just neglected to confirm that the light was flashing when he set up there and when he left. No big deal - he just couldn't prove up the essential element.