The long-term strength and serviceability of common multi-girder bridges in the United States has been the subject of considerable inquiry in the modern era, in part due to the limited resources allocated to the preservation of large populations of bridges throughout the U.S. that are approaching the end of their originally envisioned design lives. While, the conservatism that has served the civil engineering profession well for over two centuries is still appropriate for new design, in the case of aging infrastructures it has proven ill-equipped with a resulting track record of "crying wolf.0́+ Current methods of population-scale evaluation are primarily qualitative and thus struggle to effectively support proper prioritization for preservation or replacement of the large numbers of bridges built during the infrastructure expansions of the 20th Century. The disparity between what is predicted through current methods of evaluation and what has been shown by refined quantitative testing indicates that concerns over safety are largely unfounded and hence provides little evidence for the need to drastically modify current design methodologies; therefore research in this area must concentrate on strategies for understanding this safety bias and the factors that influence its behavior on a quantifiable level so it may be used as factional information by infrastructure stakeholders. The overarching aim of the research reported herein is to establish a framework whereby realistic simulations and structural identification may be brought to bear on furthering the understanding of performance of large populations of bridges. The completed objectives outlined in this dissertation include: (1) Develop and validate an automated steel girder design/modeling tool capable of developing realistic estimates of the structural characteristics/responses for broad populations of bridges. (2) Using the tool developed in (1), establish the extent to which common design assumptions can result in deterministic trends of structural characteristics within populations of bridges. (3) Using the tool developed in (1), examine how the current practice of bridge design (inclusive of the conservatism introduced through common assumptions) may produce bridges that are capable of meeting demands that were not explicitly considered during member sizing. (4) Develop and validate a streamlined parameter identification tool capable of reliably improving the representative nature of simulation models through the use of field measurements. Key conclusions from this research include: (1) Design decisions such as diaphragm type and girder spacing that are made based on arbitrary criteria can have significant influence over the actual properties and reserve capacity of highway bridges. (2) Bias implicit in conventional design processes provides reserve capacity that is critical to accommodating limit states not explicitly considered during design. (3) When incorporating field measurements within structural assessment, it is crucial to perform model updating. The non-uniqueness associated with this inverse problem can be reduced through the updating and interpretation of both global and spatially varying deterministic parameters.