Since you sort of asked for advice I will offer a little.
We all love those who brought us into this world and raised us from wee tykes and want to do right by them.
The best way I can think of to handle your situation, which may be mine in a couple years, is to treat your folks the way you want to be treated when it comes your time to hang up the keys.
With love in your heart, you must discuss the blown stop signs with your folks. It may be a relief to them or it could really send them off, but either way, you should tell them while you can. Do you really want to have to say "If only I had said something before the accident."?
I know this won't be easy, but if you are truly concerned about your folk's well-being and the well-being of those around your folks when they drive, you will do the right thing.
Best Wishes,
Brian