I have read many caselaws where the JP does not accept the testimony/comments of the defendant as they are not an expert, but in the same trials there does not seem to be any formal process that qualifies the officer as an expert.
Are officers falling under lay opinion evidence maybe?
+++ This is not legal advice, only my opinion +++
Expert opinion evidence on the other hand is only used to assist the trier of fact in understanding complicated matters. The expert is not a witness to any evidence; they are simply explaining complex matters so the judge/jury can understand the evidence and make their own findings.
Unfortunately, a lot of JP's are not well-versed in evidence law (its a VERY VERY complicated area) so they confuse the two notions. The brightest legal scholars have a difficult time discerning the differences sometimes. The rules are also always evolving.
Keep in mind though that sometimes officers are called to serve as expert witnesses----such as to opine on traffic re-constructions or forensic procedures, but they must undergo quite an elaborate questioning procedure from both sides and the court before being qualified to give expert opinion. What I'm saying is that just because they are a police officer does not disqualify them from serving as an expert on some things.
What clouds the waters most times is in the rare times that a court allows testimony to be given in both capacities---as a witness and also as a qualified expert (in the strict sense of evidence law). That's just a s**t show in balancing evidence law rules!!! In theory, they are not suppose to allow it. But in reality, if a leading noble prize winning expert just happened to be sitting next to the event and SAW everything-----their "expertise" would certainly be taken in to account by the court. They are still not suppose to give 'opinion evidence' but very few courts would not want to hear what HIS views are. Therein lies another case for the appeal courts!
But, for your regular POA court case (e.g. speeding), the officer is just a layperson who is qualified to operate the speed measuring device---they are not experts on the device. They are no different than most of us being qualified to operate our microwaves; even though we're not experts on microwave technology.