While they are relatively rare, the head on collision is the type of accident with the most fatalities. Data indicates that head on accidents account for 54% of all traffic deaths, as of 2015, but they are only 2% of all crashes. The reason is that both cars are traveling at speed in opposite directions so, the impact forces are doubled. If both cars are going 45 MPH, the impact speed is 90 MPH. This is enough force to cause serious and usually fatal injuries.
However, rollover accidents also cause a high number of injuries and are more common than head on crashes. Rollovers account for 1/3 of all road deaths in the US. This could be because more people are ejected from the vehicle in a rollover. Being thrown from the vehicle either through the windshield or a side window nearly always results in grave injury or death.