a) the purpose of police officers is not to catch the speeding people, their purpose is to protect people. The more people they can get to slow down, the better they did their job. i do not agree with the surprise thing, thats why most of the time you see officers chilling in the middle of the highway in plain sight, their job is to get people to slow down, not to slap as many speeders with citations as they can. and think about if they do go pull someone over, they now are not in plain sight of both sides of the highways, and most likely on the other side people will start speeding up agian, therefore they are not not a traffic control device while issuing a citation (only on the side of the road where they area, and we all know the traffic jams that come from someone being pulled over). sometimes you see police hiding, but more often you see them in plain sight, just because all they are doing is getting everyone to slow down.
b) heres my theory on posted speed limit - everyone knows people are going to cruise at 5-10mph over, people love speeding and hate authority, well most people. first of all you get to where you are going faster, you feel rebellious and dangerous, and you get a better rush driving faster. so i believe that speed limits are always made to allow for the "tolerance" of people to drive 5-10 over. because if the speed limit was increased to 80, people would now cruise at 85-90, just normal people not speeders (like myself, i wont lie i fly everywhere). so i honestly think speed limits (expressway, not other roads) are made to assume people will speed, and if some people dont then they are even safer (even though 70 or 80 mph wouldnt make a hug difference in a crash)