BAKERSFIELD, Calif. -- Motorcycles are the most dangerous vehicles on the road, with a fatal crash rate nearly 35 times greater than that of traditional cars.

Despite this disparity, just one state -- New Hampshire -- has no adult seat belt law, compared to 21 states that lack helmet laws for adult motorcyclists.

In 2003, Pennsylvania repealed its mandatory helmet law, and researchers at the University of Pittsburgh have published a study examining the fallout.

The numbers are startling -- helmet use among riders involved in crashes dropped from 82 percent to 52 percent. Hospitalizations due to head injuries from motorcycle crashes increased 78 percent, and the number of head-injury related deaths rose by 66 percent.

Researchers said the findings show the law was working, protecting riders from brain injury, and they note studies from other states have revealed similar findings about the impact of helmet laws.

Changes in state policy happen slowly, but safety experts said motorcycle riders can change their behavior now. They said wear your helmet, every time, because your life may very well depend on it.