A Medical College of Wisconsin study found small-overlap collisions result in increased head, chest, spine, hip, and pelvis injuries. This sort of collision is common on two-lane roads with two-way traffic where a center median is absent. Single vehicle crashes (into a tree or a pole) account for 40 percent of small-overlap crashes. According to the IIHS, 25% of frontal crash deaths are due to small overlap crashes, with the outer front wheel first to receive the impact forces rather than the more central crash absorbing structure. The IIHS has since tested family cars, compact cars, minicars, small and midsized SUVs, minivans, muscle cars and large pickup trucks through the small-overlap test. In 2017, the IIHS began conducting this test on the passenger side of vehicles. The IIHS began conducting the side impact test in 2003. In this test, the test vehicle remains stationary while a four-wheeled sled, with a deformable barrier attached, strikes the side of the vehicle at . This test is used to simulate the impact of a high-riding pickup or SUV on the subject vehicle. In 2019, the IIHS began modifying the test by using a heavier sled, changing the barrier design, and increasing the impact speed from 31 to . The IIHS cited the current test being unrealistic as the main reason for the modification. The modified test has yet to be officially conducted. In the United States rollovers accounted for nearly 25% of passenger vehicle fatalities. Features such as electronic stability control are proven to significantly reduce rollovers and lane departure warning systems may also help. Rollover sensing side curtain airbags also help to minimize injuries in the event of a rollover. In the test, which was introduced in 2009, the vehicle rests on a platform while an angled metal hydraulic plate pushes on its roof, in the area just above the windows. In order to get the highest rating, the vehicle must withstand a force equivalent to at least four times its curb weight before collapsing . In 2009, the IIHS celebrated its 50th anniversary. To illustrate how much automotive safety has progressed in five decades, IIHS tested a 1959 Chevrolet Bel Air crashing head-on, 40% offset with a 2009 Chevrolet Malibu at 40 mph. It put the video of the crash on the Internet and "the results were no surprise to anyone with a passing familiarity with cars." The Bel Air's occupant compartment was extensively damaged by the crash. Coupled with the car's lack of modern safety features such as airbags and seat belts, this resulted in the crash test dummy in the Bel Air recording forces that would have probably caused fatal injuries to a real driver.