Three questions are addressed: How many reportable accidents are in fact reported? What is the relationship among road safety, accidents occurring, and accidents reported? What are the effects of incomplete reporting on the estimation of road safety? A review of 18 studies made at different times and localities reveals considerable variability in the degree of nonreporting. As a ballpark estimate, fatalities may be known to an accuracy of about 5 percent. Some 20 percent of injuries requiring hospitalization and perhaps 50 percent of all injuries are not found in official statistics. Furthermore, the probability of an accident's being reported depends on the age of the victim; whether the victim is the driver, the passenger, or a nonoccupant; the number of vehicles involved; and several additional factors. Analysis shows that the accuracy with which road safety can be measured depends on the proportion of accidents reported and on the accuracy with which this proportion is known. It appears that the variance of the estimate of safety (or of the safety effect of some measure) is inversely proportional to the square of the average proportion of accidents reported.