Cops perform a needed function, and I think most people like that they are around. HOWEVER, the problem is that many cops have attitudes--they get on power trips and try to throw the badge around.
Another thing that causes cops to be disliked is that there are many municipalities use traffic violations to generate revenue. For example, there are numerous cases of towns dropping the speed limit suddenly in order to issuespeeding citations. This is well documented by groups like AAA--for no real reason at all, a speed limit can drop 10-20 mph with little or no warning, causing those unfamiliar with the area to inadvertently get caught in a speed trap.
A 3rd issue is that there is rarely equality under the law. Cops often drive at excessive speeds both in their official vehicles as well as their personal vehicles. Several years ago, NJ refused to increase the state speed limit to 65. Yet the same day Governor Whitman made a speech about 55mph being safer, etc, her limo and STATE POLICE ESCORT was clocked driving more than 20 mph OVER the 55mph limit. This is standard operating procedure--if you have a badge, you usually get "professional courtesy" to avoid tickets.