The premise is simple. Princess Di tells whoever it is to hand over the keys to someone who isn't driving before they head out. As a result, their car doesn't get an Close Encounter of the Pole Kind.
What would the repercussions of a surviving Princess Di be? I suspect that she and Camilla Parker Bowles wouldn't have liked each other all that much.
I wonder if Di would be elected regent if the Queen died (Charles would be ineligible because of the divorce, right?) and William took over at a young age...