Under Texas Penal Code Chapter 9 you can only threaten force when the use of force is justifiable. But by showing the weapon you threatened to use deadly force, but by law you did not use deadly force.
Sec. 9.04. THREATS AS JUSTIFIABLE FORCE. The threat of force is justified when the use of force is justified by this chapter. For purposes of this section, a threat to cause death or serious bodily injury by the production of a weapon or otherwise, as long as the actor's purpose is limited to creating an apprehension that he will use deadly force if necessary, does not constitute the use of deadly force.
in short your just letting your road buddy there know that "if you insist on continuing this behavior I will ruin your day" - perfectly legal as long as your not the one who initiated the drama.