Serious? Besides sight issues and relativity? Even before radar, cops didn't simply guess. They used old scientific methods utilizing a stopwatch and two known points, lines in the road, pacing etc. All of these have been proven to be innaccurate(due to manipulation of stats and the mathematical imcompetence of the general populus) of, hence the creation of radar, laser, etc. to give a semi-dummy proof scientific method of measuring speed.
This is exactly the reason why I have surveillence equipment installed on my car, to help weed out cops who think they have some supernatural gift of measuring speed etc. Hell, this was all sparked when I was a "punk kid" and was being harrassed by the local PD.(well it was two cops, but still) needless to say I simply gathered evidence, and pressed charges for harrassment. it never made trial(semi-settlement), but the PD did apologize and dismissed the police Officers from duty. (Fired) they all dropped all traffic citations written by the two cops, valid or not. (40+)
Who is a cop, trained or not, better than any person living in the united states? Their testimonies should be taken with a grain of salt, just like everyone else. First hand testimony has been proven to be unreliable at best, hence the need for scientific anaylisis and evidence.
And on a side note, some(most) police forces do have a quota system, and that's how they get promoted. If there were no motivation for police to write tickets, well, there would be a fraction of written tickets there are now. I know all this first hand, as I have friends who are lt.'s in the chicago police force.