I think Bob's idea is really interesting, if a little creepy. Ultrasonic speakers playing wolf pack noises? I mean, that's just so distopian.

Also, cars are so ubiquitous now, if you made that type of sound ubiquitous, too, wouldn't it just stop working? The deer or whatever would be hearing that so often and from so many directions, without ever associating it with any of the usual consequences of that noise (ie: deadly peril) that the effectiveness could wear off, and you might end up confusing the animals enough in wild areas that they'd end up being easier prey for real predators. Just a theory, though. Besides, we'd have to have a way to turn it off, because you wouldn't want that in national parks, messing with the natural rythyms and scaring away the wildlife that visitors come to see. Though I expect there's a fair amount of roadkill in national parks, so it wouldn't be terribly effective.