"You're safest on limited-acess 4-lane if you drive about 6% to 8% faster than traffic in general. This was analyzed by the Society of Automotive Engineers, long ago."
Agreed. Then you can "wipe" cars as you pass them, keeping a really good mental picture of the space on all sides of you vehicle. You also get to see what the crazies are doing before they do it. If I am going somewhere fast, I vary speed a lot, the open mile or so between the clumps is pretty wide open, then coast off to clump speed as I near the next one. How you come up on the road turkeys really does determine how soon you get passed. If you are a few hundred yards back and approaching at 5mph advantage everybody and their brother will step out to pass. If you have a little more aggressive approach speed people tend to stay put. If I have to step on the brakes to get to clump speed though I am not doing my job of paying attention. If not going anywhere you can ride for 40 miles in between a clump, seeing very few cars. It most certainly is a mental thing, people are demonstrating their herd mentality.
As for the mirror thing, adjustment is easy. Sit in driving position and lean your head on the side window. Adjust your mirror until you barely see the edge of the car. Lean the same amount right and adjust the passenger mirror. A passed car should move seamlessly from peripheral vision to side mirror to rearview, no gaps. A lot better than the driver's Ed. mirror technique, everyone pretty much learns to duplicate their rear view picture with their side view.