Billboards are a type of collectible objects present in the Need for Speed Unbound game. To destroy the vast majority of them you will have to gain enough speed and jump up to hit the billboard. Mostly you will find some kind of ramp, mound, hill, overpass, etc. nearby that would help you do this. After destroying each of them, you will receive $250.
There are 80 billboards in total, and when you find and destroy them all you will unlock a new car - Ford Focus RS 2016, and earn the "Adbusting" achievement / trophy.
