The Motorcyclist Anthropometric Test Dummy (MATD) and injury risk/benefit analysis methods standardized under International Standard ISO 13232 allow the relative injury benefits and risks of rider protective devices fitted to motorcycles to be assessed, for a specific set of injury types. Research involving the feasibility of airbags fitted to motorcycles intensified the need to upgrade the crash test dummy neck injury assessment methods. This involved the development of an improved dummy neck with multi-directional biofidelity and injury assessment capabilities and corresponding probabilistic four axis neck injury criteria. The neck injury criteria were developed by fitting the distributions of neck injury severities observed in onscene in-depth investigations of 568 real-world motorcycle crashes, including the direction of neck motion indicated by special detailed neck dissections in 67 fatal cases, to the distributions of upper neck forces and moments measured in calibrated computer simulations of the MATD with the improved neck in the 568 crashes. The result is a probabilistic injury criterion that can estimate the probability of neck injury, based on four axis upper neck forces and moments measured with the new MATD neck. The model has a high level of overall agreement with neck injury severity levels and directions observed in real world crashes.