The law is that each driver is responsible for safely operating their own vehicle, so whichever driver failed to do so, thereby causing the accident, would be at fault. If the rear driver was stopped at a signal and the front driver backed his vehicle into him, the front driver would ordinarily be at fault. If the front driver was stopped at a traffic signal and the rear driver just plowed into him, the rear driver would be at fault. The only thing that makes rear end accidents tricky is not the facts of who is at fault, but the evidence. Most drivers do not operate their vehicle in reverse when driving
on the roads, so when a rear end accident occurs, the rear driver is almost certainly the one who is at fault; if you have a rear end accident and the conflicting testimony
of the drivers, the rear driver will almost certainly be found liable based solely on the physical evidence; it's not an issue of law--it's a question of evidence.
In cases where more than one driver is at fault, a driver can recover in Indiana only if it is determined that he is less than 51% responsible for the accident. So, using the traffic signal example, perhaps the front driver was fiddling with his radio and stopped on his brakes rather abruptly for the traffic signal; let's say that the driver behind him also wasn't paying attention and, partially because of the front driver's sudden stop, there was a rear-end collision. The front driver might be found 20% or so at fault and the rear driver at 80%(ish). the front driver could recover damages, but the rear driver could not even though the front driver contributed somewhat to the accident because the rear driver's own liability exceeded 50%.
Let me know if further clarification is needed. Thank you.