Over the past six years, U.S. drivers have had fewer crashes. But when they do crash, subsequent medical expenses and repair costs have gone through the roof. Those two trends have conspired to keep insurance premiums relatively stable—for the time being, at least.

“The continued drop in claim frequencies has offset escalating car repair and medical care costs,” says Elizabeth A. Sprinkel, senior vice president of the Insurance Research Council. “However, if claim frequencies start to rise or even just stop declining, then rising claim severities will increase loss costs, creating upward pressure on premiums for consumers.”

Sprinkel bases her conclusions on trends in auto insurance claims over the last six years. On one hand, statistics show declines in the frequency of claims for property damage (down 11 percent), bodily injury liability (down 19 percent) and personal injury protection (down 14 percent). That jibes with national trends showing declines in injury and fatality rates, indicating that roads and vehicles are getting safer. On the other hand, the severity of claims increased for property damage (up 2.9 percent per year), bodily injury (up 3.3 percent per year) and personal injury protection (up 2.9 percent per year.)

In 2006, in fact, claim severity reached a 15-year high. But claim frequency reached an all-time low, which kept premiums in check.