Economic Evaluation of Bridge Seismic Retrofit Improvements

Travel-related and damage-avoidance benefits and costs of conducting seismic retrofit improvements to arterial bridges are examined, with consideration of the risk posed by a relatively low probability but high damage earthquake (a design level event). The city of Seattle’s arterial bridges and viaduct structures serve as vital components of the urban road network, providing for the ongoing transportation needs of citizens and commerce. Failure of these bridges in the event of an earthquake would have significant negative impacts for the movement of people and goods, and thus on the local economy. Shoring up the city’s bridges to resist earthquake damage would clearly bring benefits, thereby preventing the impassable arterial bottlenecks that would otherwise occur. The question is whether or not the potential benefits of seismic retrofit improvements warrant their investment costs, thereby providing an economically efficient use of public dollars. For the purpose of addressing this question, a methodology for evaluating the potential benefits and costs associated with bridge seismic retrofit improvements is presented, quantifying these impacts for a two-phase retrofit program and considering the results in light of risk and uncertainty. The evaluation procedures developed consider a major earthquake with defined probability and compare the travel-related and damage-avoidance benefits that would be generated by retrofit improvements with their associated implementation costs. Emphasis is placed on travel impacts because researchers would find nontransportation economic impacts extremely difficult to quantify without knowing all of the other physical, built environment impacts that would occur with a major seismic event. Standard measures of economic feasibility are reported, and in the case of Seattle, seismic retrofit improvements demonstrate moderate positive economic rates of return. The implications posed by expected utility theory for risk-averse decision makers in such cases involving earthquake uncertainty are also discussed.