Jorian, he was not tased for speeding or refusing to sign. He was tased for ignoring the cops orders to put his hands behind his back and then try to walk away. Let's keep that straight.
RickM, I don't know where this event happened, but in most places,
AFAIK, you DO get arrested for refusing to sign. You have to get out of the car in order for them to arrest you. In fact, I've been asked to exit the car and go back to the cop car even when I wasn't getting arrested. Lots of cops do that to keep from having to stand in a busy road.