I was looking for something like that, but I didn't expect it to be in a behaviour. Finding out if something is moving seems like a pretty standard thing to me. It should be in the standard list of conditions.
Plus, I wanted to spawn the tracks at a different rate depending on whether the tank is moving forwards, backwards, or turning (these things happen at different speeds).
I know it's not the most elegant system - I really did knock it out it 15mins. I'd love to see how other people solve the same problem.