Over on our Bay Area ride http://www.vfrdiscussion.com/forum/index.php/topic/71386-may-26th-bay-area-roll-call-ride-xxx/, we came upon a couple, he was on a V-Strom and I forget what she had and they were 10-15 UNDER and slowing up an Oldsmobile up the road to Mt. Hamilton. They wouldn't take a turnout and only 3 riders passed them and the Olds did they wave the rest by.
I felt sorry for the lady because she was locked-elbow, pointed straight ahead on a seriously technical road going from 0 - 4200ft. They made it though. We saw them at the observatory.
If you have to drive slow to be safe, pull over, wave us by, but do something to acknowledge your current lack of skill or ridiculously leisure pace.