Driving a lot on interstates, in a place where passing on the right is common (Ark), I have noticed this problem a lot more than most folks. As I've said a few times, I think this is the worst feature of the Prius at least as to safety. (i.e., I'll bet my chance of getting into a really bad accident is higher because of this than because of either sudden unintended accelleration or brake intermits on bumps.) I bet that Toyota could make the rear window wider and the rear side windows go further back without affecting the structural integrity of the car. This would reduce the physical blind spot and (by increasing the view of the rear view mirror) the mirror blind spot as well. While I have learned a lot about using mirrors from driving the Prius, I don't think it is good to assume that everyone will always set their mirrors optimally and will always drive optimally, both checking physically and in mirrors. I screw up on each occasionally, and I bet most other folks do too.
Yesterday I was merging from a highway to a road and had to look toward the right rear. I could not use the right mirror as the angle was not right to see the traffic. I really could not see anything when I looked toward the right rear corner. I actually had to stop to make sure there was no traffic. These kinds of merges happen a lot around NY.
Rear right visibility for the driver is poor. And rear left visibility for the front seat passenger is poor too. It's because of those huge triangles in the rear corners of the Prius created by the styling of the "C" pillars and the slope of the rear hatch. Because of the slope of the rear hatch and the side window lines, you might not notice these huge triangles from the outside. But inside, e.g., when you quickly look back toward the passenger side rear corner before changing to the right lane, these rear corner blind spots are huge.
maximum rearward visibilty. My guess is many people do not get much out of their mirrors because of the way they adjust them. I drove a Bobtail truck many years ago and the company had no problem allowing any of the employees to use it whenever they wanted. Seemed like every Monday morning when I would get into the truck, the mirrors would be adjusted so that all you could see was the side of the truck. The side of the vehicle is always in the same place so no need to look at it in the mirror. I adjust mine to show the smallest amount possible of the side of the vehicle. I have seen that link before and it really does a good job explaining how to set up both side mirrors to work with the inside mirror. One of those things that should have been discussed in my driver training but was not.
