were wearing a helmet, which were estimated to give a 37% increased chance of overall survival and reduction in the chance of a head injury; helmet use has been the subject of various studies and laws. In 2013, 6% of the fatalities were passengers (non-operator) of the total deaths that year for motorcycles 4,688. Ridership and demographics From 1980 motorcycle ownership among riders aged 40 and over increased dramatically, from 15.1 percent of all riders in 1980 to 43.7 percent in 1998. The mean engine displacement of the motorcycles involved in fatal crashes also increased, from an average engine size of in 1990, to in 2001, an increase of 24.7 percent. It has been suggested that the combination of older riders on higher-powered motorcycles might have been partially responsible for the increase in motorcycle deaths from the late 1990s until 2004. Blunt abdominal trauma can be caused by a motorcycle accident, and can damage the testes and intestines. Half of motorcycle fatalities in single-vehicle crashes relate to problems negotiating a curve prior to a crash—almost 60 percent of motorcyclist fatalities in single-vehicle crashes occur at night. However, even when riders survive, around 10% of trauma injuries include damage to the genital area, such as due to the collision of that area with a bike's gas tank. In 2009, motorcycle fatalities in the US declined for the first time in 11 years but remained above 4000; that year's total dropped from 5,312 to 4,469 (between 2008 and 2009). Automobile fatalities continued to decline for the seventh straight year before 2011. A decline in recreational motorcycling due to the late-2000s recession might account for the decrease in accidents, according to the authors of a report by the Governors Highway Safety Association (GHSA), but a state motorcyclists' rights organization, the Motorcycle Riders Foundation, said motorcycle use appeared to have increased, influenced by motorcycles' better fuel economy. A preliminary report by the GHSA estimated that the number of motorcyclist deaths in 2012 was more than 5,000, a 9% increase over 2011. In the USA during the 2010s, motorcycles comprised about 2% of registered vehicles (about 6 million bikes registered), but 14 percent of all fatal accidents. In addition, the rate of accidents per mile has gone up from the 1980s and was not due to increased ridership. One study evaluated motorcycles as being 37 times more dangerous for a fatal accident than a car.