One of the main things that must be done after a car accident is to determine who was at fault. From a legal perspective, the most important document is going to be the police report. The officer who responds is going to do an investigation -- for major crashes, an entire investigative team could be sent in -- and decide who caused the crash. This information can then be used as the basis for lawsuits, insurance payments, tickets and things of this nature, so it is important to understand how it works.
When it comes to rear-end crashes, the rule of the thumb is that you are almost never at fault if you are in the front car, the one that was struck from behind. There are exceptions to every rule, so it is impossible to say that the front driver will never be at fault, but the rear driver is going to be blamed in the vast majority of cases.
This holds true even if you, in the front car, decide to do something unexpected. For example, you may see a deer and come to a stop in the middle of the highway. While watching the deer walk into the woods, your car could get hit from behind because the other driver never expected someone to be stopped at a place with no stop sign and no light.
While that driver may be annoyed and feel like it is your fault for stopping, it really is not. Driving rules dictate that trailing cars must always have room to stop if the car in front of them slams on the brakes without warning. Drivers who do not allow for this much space could even be cited, in some cases, for reckless driving.
Source: FindLaw, "Car Accident Liability: Proving Fault in a Car Crash" Oct. 15, 2014