Back Seat Granny Driver

It's no secret: I drive like an old lady. I know it. I've dealt with it. I embrace it.
I have certain physical limitations that have forced me to make adaptations when I am driving... mainly the fact that I have very VERY poor depth perception makes driving difficult. Driving at night is nearly impossible for me. But I know my limitations, and becuase of this, I drive 'slow' (never more than the speed limit) and allow myself 'extra space' (when parking, following, or merging... whatever) Slower speeds and more space offer me more reaction time, therefore make me a safer drivier despite my physical limitations... but it also means sometimes I drive slower than the people I share the road with... or even the people I share a vehicle with.
At the risk of really sounding like an old lady fuddy-duddy... I want you to know that I do not drive over the speed limit. And I don't really like riding in vehicles where the driver does. I know, I know... You are sitting there thinking, well, I only drive 5-10 miles over, and they are not going to give you a ticket for that and I am a safe driver, so it's fine. And perhaps you are an excellent driver... But Joe-Schmo driving next to you on the interstate is not. And when Joe-Schmo makes a bad decision that threatens your safety that 10 mph over the speed you should be driving has just drastically reduced the time you would have to react to Joe cutting you off or slamming on his brakes... etc. Besides, 5-10 miles over the speed limit is only REALLY going to shave a matter of seconds off of your arrival time, though it increases your risk of accident and/or injury exponentially. To me... not really worth it.
So when I am driving, I don't get in a hurry. I make it a habit to allow myself plenty of time to get to my destination so 'hurrying' is not a temptation, and I stay as calm and focused as possible while driving.

Anyway, I am thinking about all this becuase the other day I was in a vehicle with a family member who was driving in such a way that I felt unsafe and uncomfortable. There was a small case of road rage in which the driver of the vehicle decided to tail another driver at a greatly increased rate of speed (in excess of 15 mph over the posted speed limit) on a two lane highway. There was little shoulder on either side of the road, and little room for error. I was exceptionally nervous.
It's pretty universally understood that no one really appreciates a back seat driver. So I closed my eyes and gripped the door and prayed that nothing drastic would happen. We arrived safely. But it kinda made me wonder. When, if ever, is it appropriate to comment on someone else's driving?
Is there ever a time when it is appropriate for someone beside the driver to make a call on how fast someone should be driving for the the situation, road condition, safety of others in the car, etc.?
Is there a time when it is appropriate to say to a driver, your actions or attitude are impairing your ability to drive, you need to cool down/focus or allow someone else to take over? (And I am not talking about someone who has been drinking, lets all just agree that you would never let them begin to drive... but I have told my husband before not to eat, or flip through his CD's while driving... my sister is the queen of texting while driving--which scares me to death... or the case the other day with the road rage impairing the driver's judgement...) At this point should you as a passenger say-- for the safety of myself and others, you need to slow down/cool down/ take a break/put the phone down...
OR
Do you accept their driving and judgement when you get in the car with them and respect their space and keep your mouth shut?
What do you think? Is there a line there? Where would you draw it?