Transportation systems are spatially distributed systems whereby components of the system are exposed to different ground effects due to the same earthquake event. The ground effects that various components of the system are subjected include ground shaking, vertical displacements due to settlement, and horizontal displacements due to lateral spreading and sliding. The ground displacements occur because severe ground shaking causes liquefaction and landslides under the appropriate environmental conditions. Bridges are key components of transportation systems and are particularly susceptible to liquefaction and landslides as they are located over streams and rivers with piers situated over sandy saturated deposits; or they may be over canyons with high slopes that may result in slope instability. Thus, it is important to integrate the effect of local site conditions in the overall earthquake risk of a transportation system.

Consideration of the spatial dependence of individual components is an important factor in the evaluation of the network system connectivity and traffic flow through the system. Risk assessment methods require that not only the component performance is assessed, but the overall system performance is evaluated. Most recently, Werner et al. (2000) and Basoz and Kiremidjian (1996) considered the problem of transportation network systems subjected to earthquake events. In both of these publications, the risk to the transportation system is computed from the direct damage to major components such as bridges and the connectivity between a predefined origindestination (O-D) set. Basoz and Kiremidjian (1996) also consider the time delay and use the information primarily for retrofit prioritization strategies. The current software HAZUS (1999) for regional loss estimation developed by the National Institute for Building Standards (NIBS) for the Federal Emergency Management Agency (FEMA) considers only the direct loss to bridges in the highway transportation network. The connectivity and traffic delay problems resulting from damage to components of the system are not presently included in that software. Chang et al. (2000) propose a simple risk measure for transportation systems to represent the effectiveness of retrofit strategies by considering the difference in costs associated with travel times before and after retrofitting.

In this article, a method for risk assessment of a transportation system is postulated that considers the direct cost of damage and costs due to time delays in the damaged system. The method is applied to the transportation network within five counties in the San Francisco Bay Area and conclusions are drawn on the basis of the application. The site hazards considered in the direct loss estimation include ground shaking, liquefaction and landslides. The effect of bridge damage from ground shaking hazard on the transportation network is studied under the assumption that traffic demand following the earthquake is either constant or variable.