Property Damage

Data for property damage crashes is even more uncertain than for injuries. In some jurisdictions the criterion for reporting is damage exceeding some monetary amount specified by statute. Because of inflation, this requirement may include more and more minor crashes as time passes, until the amount is abruptly changed, thereby reducing the reported number of crashes. Drivers generally report single-vehicle property damage crashes only if they see some benefit in reporting them, regardless of legal obligations.

Source: http://en.wikipedia.org/wiki/Road_safety