One of the most common crash modes on US roadways are rear end collisions. Despite this large number of crash occurrences, rear impact crashes are generally viewed as a benign crash mode primarily involving only property damage or at most minor injuries. However, in 2015, there were nearly 1,000 fatalities in rear struck passenger cars and light trucks;​ 757 (63%) of these fatalities occurred in model year 2000 or later vehicles. The question becomes, if rear impacts are indeed a benign crash mode, what is leading to these fatalities? This study investigates the factors associated with serious‐to‐fatal rear impact crashes. Compartmental collapse appears to be a major risk factor in rear impact crashes, particularly for occupants seated in the rear seats. The vast majority of fatal crashes appear to involve compartmental collapse, which is present in 4.4% of all rear impact crashes, although other injury mechanisms such as direct head contact to rigid interior vehicle structures and thoracic loading from the seat tend to be responsible for serious but survivable occupant injuries.