If you are worried about your safty when driving with the top down then I would suggest getting a firearm. I am trained in several Martial Arts but I still carry a gun because I know the law is better for gun CCWers in self defense than "Fighters" in self defense. The line between self defense and assult is too thin when fighting but a gun shot is always self defense unless you shoot the guy in the back.
My personal choice for a life saver :
FN FiveseveN 5.7X28mm
Glock 19 9mm