What is a good knife to use for self protection?

I am going to college next semester and the area where i am moving into is pretty rough. There was already two armed robberies in the past week so i was thinking i should buy a knife just to keep in my car or pocket if i am ever alone. I dont need some big rambo knife or anything like that, just something to keep me somewhat safe in the event of a robbery. Also something under 100 bucks would be preferable.