This report examines the equivalent base length concept used for the development of the Ontario Highway Bridge Design truck. As used in Ontario, the concept does not rely on the assumption that a vehicle can be described by two variables, the weight W and equivalent base length BM, but rather that a vehicle can be described by a series of points in W, BM space. This report defines an alternative equivalent base, called the concentrated base length, b, and introduces a location parameter, x. The validity of the base length concept is demonstrated by developing a vehicle equivalent to a hypothetical family of Australian legal vehicles. The accuracy of resultant curves of design bending moment and shear is assessed. It is concluded that reasonable accuracies can be achieved by the use of a single, non-variable design truck, and that the Ontario method is useful and important.