The problem of relating driving performance to the probability of accident has been handicapped by the lack of an intrinsic performance criterion, one which can serve in a manner analagous to tissue damage or unconsciousness as physiological criteria. Response blocking is proposed for this purpose since it is characteristic of continuous, high information rate tasks, is involuntary, is apparently subject to all of the factors which affect driving skill, and is necessarily related to accident probability. The characteristics and possible uses of response blocking are discussed and illustrated in the context of a recently-developed approach to the quantitative prediction of performance under environmental stress.