Re: Speeding
Legend: I think you may be worrying unduly about the dangers of third party reporting of bad driving. In my case, I only report the most blatantly bad driving. For example, the last time I did so was when a young fellow passed me and five/six other cars all in one go on a relatively windy road. There was a hard shoulder, so his logic must have been that if he met someone coming the other way they could pull in. Fine until the day that he meets someone as stupid as himself coming the other way. If there is some room for "interpretation" as to what is or isn't dangerous, I wouldn't bother.
Of course, there is always the danger that someone will make a malicious/false report. However, that is where the presumption of innocence comes in. Ultimately, the credibility of the witness then becomes an issue. That is why I always make it clear that I am happy to make a statement/testify if the guards want (though it has never come to that - as already mentioned, I usually just ask that they have a word)
Realistically, if a guy is driving like an idiot, he is going to be spotted by a lot of different drivers. So if people were more willing to report bad driving, it would be very easy for the guards to spot the reallly bad offenders, because they would be the subjecct of multiple independent complaints. If everybody reported just, say, the two or three worst offences they saw each year, this would have a huge bearing on road safety.