Compared with cars, motorcycles are an especially dangerous form of travel. Per mile traveled, the number of deaths on motorcycles in 2003 was about 32 times the number in cars. Motorcyclist deaths have been rising in recent years — up 89 percent between the all-time low in 1997 and 2004. Motorcycles often have excessive performance capabilities, including especially rapid acceleration and high top speeds. They are less stable than cars in emergency braking and less visible. Motorcyclists are more prone to crash injuries than car drivers because motorcycles are unenclosed, leaving riders vulnerable to contact hard road surfaces. This is why wearing a helmet is so important. Helmets are the principal countermeasure for reducing crash-related head injuries, the leading cause of death among unhelmeted riders.