In this paper, a previously developed probabilistic fracture mechanics model—based on the Eurocode fracture mechanics design method—is used to assess the effects of climate, traffic conditions, and bridge configuration on the probability of failure by brittle fracture of Canadian highway bridges. The purpose of this assessment is to determine the relative importance of these parameters and provide insight that may be relevant if this model were to be used to calibrate the toughness requirements for the design of new steel highway bridges. Based on the presented analysis, it is concluded that the effect of the bridge location is relatively small, when three locations in each of Canada’s two climate zones are compared. The influence of traffic volume on the probability of failure by brittle fracture for average daily truck traffic (ADTT) volumes between 50 and 4000 trucks is also found to be relatively small. Similarly, bridge span and influence line are found to be relatively unimportant. Of the parameters investigated, the most important one is the effect of multiple vehicle presence (i.e., simultaneous truck crossing), which is found to be potentially significant when high traffic flow rates on long bridge spans are simulated.