Depends why? If you're always speeding because you are actually trying to hurt yourself then, yes, this is "suicidal behavior." if you are doing for any other reason, for example, because you're late, you like the "rush" of driving fast or you're just showing off for friends, then that is not "suicidal behavior." bottom line, it's the "why, " not just the action itself that defines if you are suicidal or not.
Sometimes. Speeding could be a symptoms of other behavior like manic or impulsive behavior.