Einstein asked this very question, more or less. This is why nothing can travel at the speed of light. What happens is that the headlights shine ahead at the speed of light. If the vehicle is traveling near light speed, the next question is relative to what? As far as you sitting in that vehicle goes, you're standing still and the headlights are doing their job, illuminating what's in front of you. The speed of light is constant, regardless of the reference frame. If somebody saw you zipping by at nearly the speed of light and saw you turn on your headlights, the observer would see the light from your headlights moving at the speed of light, as would you, regardless of your state of motion.