All to overcome the opinion of one cop. That's a rigged system and you must know it, my FRiend.
Considering the training and experience that the one cop must accumulate to have his professional opinion and estimate carry weight in court, I believe it's a matter of him earning it.
The fact is, the one cop might spend 8-12 hours a day, 5-7 days a week watching cars come and go, and probably gets constant feedback from a radar gun to train his speed estimation. That's how it worked for me anyway. It's simply a matter of hours and hours of practice and training. You really do get to where you can estimate it with pretty good accuracy.
If you have complaints about the way the courts work in general, in terms of the burden of proof and the weight a LEO's testimony carries, I'm afraid that's a bit outside of my field, and probably outside this thread too.