This time I got a reply:
Your idea is called dynamic carpooling, or "on the fly" carpooling. The technology exists and in theory such a system could be established. The problem is human behavior and instutions -- people are unlikely to share rides with other they don't know (personal safety, driver behavior); there are huge liability problems; the added inconvenience is generally not worth the dollar savings.
That's sad to hear...but I couldn't really believe it. Happy to hear that my idea, although not original, had a name, I decided to see if anyone had tried it...after all, what would an expert in the area know anyway?