The cop isn't playing anything safe, nor does he have any incentive to. He wrote the actual measured speed. Even if the defense showed an error, the court will still find guilty if the speed in favor of the defendant is greater than the limit. Contrary to all the ill-motives you give the cops, they do cut people breaks. There is no reason to under charge this other than to give the guy an out on points. The various agencies involved get the same surcharges whether he had been charged for 28 over or 5 over (and most of those surcharges are state mandated things rather than going to the "department" the officer works for... some of it gets sent to EMS and some gets put in a vicitms of crime compensation fund... it's amazing all the details on a PA citation).