The development of the Head Injury Risk Curve (HIRC) and the Skull Fracture Risk Curves (SFRC) that were proposed by Prasad and Mertz for the adult driving population are reviewed, and the problem with using the Maximum Likelihood method to analyze the cadaver 15ms HIC data is discussed. The cadaver data base for skull fracture is expanded by including non-fracture 15ms HIC values for a number of cadavers which had skull fractures at higher impact severities and by including cadaver test results of Ono and Tarriere. This expanded data set was analyzed using the Mertz/Weber method and a new method called “Certainty Grouping”. An updated version of the Skull Fracture Risk Curve (SFRC) is proposed. The efficacy of this revised curve is demonstrated by comparing its predictions to the results of simulated fracture impacts using a finite element model of the head. The HIRC was not changed since no additional brain damage data were analyzed. The efficacy of the HIRC is demonstrated by comparing its predictions of head injury reductions due to the introduction of airbags and due to improvements in football helmets. The cadaver data of Nusholtz and Stalnaker were added to the expanded cadaver skull fracture data. These data were analyzed to determine the relationship between peak resultant head acceleration and the risk of skull fracture. For equal risk of skull fracture, the 15ms HIC criterion is more discerning because it is time dependent.