Car Accident: Rear End Collision, Who is Liable?

The laws vary from state to state, but generally speaking the driver of the car that hits the other car is presumed to be at fault; in this case, Driver 2. There's no question that Driver 1 in this scenario did something illegal, but the motor vehicle code usually says that all drivers are required to maintain control of their vehicles at all times -- hence the presumption that you are at fault if you hit someone else. It would be very difficult for Driver 1 to be named as the at-fault driver unless a third party witness (like someone driving another car) saw the accident and reported the details to the police, and even then the police officer may not find Driver 1 at fault.