Check For Collisions With Boundaries? Help

Hi, i've just started out with Unity 5 and am making made a "Crossy Road" type game to get to know the engine a bit better and can't figure out how to make 3D objects check for collisions to stop the player from going too far left / right and clipping through the boundaries. the boundaries are along the side of a randomly generated road and are generated every time the "up" arrow is pressed. Any help would be appreciated.