TKD wouldn't be my first recommendation for self-defence, but frankly, most traditional martial arts training won't prepare somebody very well for a real altercation. (There can always exceptions, to be sure.)
That being said, not long ago, I watched some Youtube videos of real situations, and, lo and behold, I witnessed some advanced TKD practitioners accomplishing one-hit knock-outs by a kick - even spinning and jumping ones! Okay, they were not facing highly skilled opponents - aggressors on the street usually aren't. Anyway, I was reminded that there are just no absolutes in combat. Or anywhere else, for that matter...