I am not sure in other states, but the law in Pa is on highways (and that isn't just limited access highways) you are to be travelling ay a speed consistent with the flow of traffic, which generally will not be the speed limit. Travelling just five miles-an-hour slower than everyone else is presenting a serious danger to the travelling public around you. In addition, 83% of people generally travel at least 5 mph faster than the limit. For those who spend a large portion of their time on the roads, this is self-evident.
For that reason, cops generally don't pull someone over just for "going over the speed limit"; he'd have to ignore the other 40 vehicles traveling at the same or greater speeds. If a cop does pull someone over for "doing 45 in a 35"-especially when said vehicle was in a large flow of vehicles and was traveling with with them, his motive is rightfully questioned, and if the person would simply challenge it in court, they would get it overturned, but who wants to do that, especially with an out-of-state ticket (which they generally are counting on when they present bogus tickets like that.)
Please keep in mind, the speed limit on the highway has nothing to do with safety; that speed is a relic from efforts to curb fuel xonsumption during the oil crisis from the 70's-a mission, it should be noted, that it failed miserably at, just as it fails miserably at preventing accidents and fatalities today; the only thing it succeeds at is generating revenue for the township/county.