A method for calculating the probability of wheel lock occurring during braking with passenger cars is presented. The method combines (1) the probability distribution of vehicle deceleration during braking, (2) the probability distribution of tire-road friction exhibited by the general road system, and (3) the braking efficiency level of the vehicle, to predict the probability of the occurrence of wheel lockup during a given braking event Results are presented employing the constituent probability data available in the open literature. These results predict that lockup events are very rare for typical drivers operating vehicles of 80% braking efficiency or better. As braking efficiency falls, the frequency of occurrence of lockup rises rapidly. At braking efficiency levels as low as 50%, typical drivers would experience lockup about once a month or once every 35 miles of travel on wet roads. A general observation is made that exponentially rising rate of failure is to be expected as system performance qualities degrade from the norm. Findings of several other researchers are noted which confirm this general observation.