Is it my business or shall I just butt out?

My 92 year-old grandfather is a wonderful man with his faculties mostly intact, but because of his age, hearing, and reflexes, he is not a safe driver. He is a danger to himself and others and while most of his grandchildren would very much like to see him stop driving, none of his children has stepped up to take away the keys. I mentioned this to my father (his son) who got angry and told me that taking his license would be killing him because without that freedom he would decline and die within a very short time. I told him I wanted to have the DMV call him in for a test and he kept telling me to think about the bad karma involved for me in doing something like that. So, do I make the call or leave it to my grandfather’s children?

Addendum: I think my sister or I has asked this question before, but since his wife died last week, my grandfather is very distracted and this will certainly exacerbate the problem.