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, and an improved dummy neck with multi-directional biofidelity and injury assessment capabilities and corresponding probabilistic four-axis neck injury criteria (upper neck axial compression and tension forces, lateral bending, extension and flexion, lateral bending, and torsion moments) were subsequently developed. The four-axis neck injury criteria originally proposed for ISO 13232 had a “trapezoidal egg” shaped injury index, based on mechanical stress ratio principles, which tended to under-predict injuries under tension-only loading conditions, compared to injurious tension force levels reported in the technical literature. A revised neck injury criteria was then developed having a “clipped trapezoidal egg shape” index that is similar in concept to the two-axis “clipped kite” shape criteria specified by the US Motor Vehicle Occupant Crash Protection Standard (FMVSS 208). The improved Neck injury criteria were developed by fitting the distributions of neck injury severities observed in on-scene in-depth investigations of 565 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 565 crashes. The improved injury criteria can estimate the probability of neck injury, based on four-axis upper neck forces and moments measured with the new MATD neck with a higher level of overall agreement with neck injury severity levels and directions observed in real world crashes, compared to the previous four-axis criteria.