In many of the United States, there is something called the Advance Directive, which is a legal statement that you (1) do or (2) don't want to have "heroic measures" used to keep you alive when you're in a coma or otherwise unable to advocate for yourself.

These forms also include wishes for post-death disposition of your body. It makes it easier for your survivors/loved ones to know what your wishes are, and gives them legal clout to make sure that hospitals, etc., follow your wishes. You give copies to your doctor, your spouse, etc.

These forms might be a more clear way of expressing your donor wishes than those drivers license donor cards.