The investigation of the effects of impact on the members of highway bridges is a considerably more complicated matter and the results less susceptible of accurate interpretation than in the case of railway bridges. In the latter it is fairly well established that the principal causes of impact are those due to unbalanced driving wheels and cumulative vibration at and near the critical speed of the travelling load. Such secondary causes as uneven track, defective rail joints, wheel tyres worn out of truth, pitching and rolling of engines, are largely capable of elimination by reasonably careful attention to maintenance and design of track and rolling stock. In the case of highway bridges, the wheel loads do not travel along well-defined lines nor vertically over individual members of the floor framing. The imposition of the load comes most frequently on some casual point or small local area of a floor panel or slab, and is thence transmitted to the floor members proper. The type of floor thus enters largely into the question, whilst relative smoothness of floor surface, possible obstructions, character of traffic and length of span to be loaded for producing the maximum effect further complicate the problem.