Tradeoffs are different between computers and humans. Defensive driving is about trying to stay within an envelope where you can safely respond to whatever happens. A computer system will have a different envelope, so it won't need to do the same stuff.

To make a terrible analogy, a human pilot flying by eye needs to eventually make a decision to land at an alternate airport when his primary is fogged in. Is a computer smart enough to make that judgment call? Well, if the computer is equipped with a zero-zero landing system and can safely land in the fog, who cares?

Your question about rear threats is intriguing. The proper response is a lot less clear there, I think. Do you speed up? Slow down? Change lanes? Squirt them with washer fluid? Drop some caltrops from your rear bumper?