After a while you will start gaining a relationship with some of the other drivers. Pretty soon you will have the option to go to one of there races or do a throw down against them. Do any of these and you will soon be come true friends with them and it will unlock. Check under My Stuff, Relationships for the status on each driver of what they think about you. If it says they think you're a legend, then you've maxed them out. Do this with every other driver to unlock the achievements below. This cannot be missed.