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Retrofitting existing buildings (e.g., anchoring houses, bracing walls) can be expensive, and many owners do not have the funds for compliance even with minor strengthening requirements. Thus, a policy of selectively retrofitting buildings on the basis of relative risk may be appropriate. For example, in the case of unreinforced masonry buildings, Durkin and Thiel's research shows that many injuries in recent California earthquakes have occurred outside the buildings, often to occupants attempting to evacuate (31,79,98). This finding suggests that protecting the evacuation route out of URM buildings and along the buildings' perimeters may yield substantial reductions in the number of injuries and deaths at a modest cost (99). Other relatively simple modifications that may increase the probability that severe damage will cause fewer injuries include strengthening stair wells or bathrooms or creating "safe" corridors (95).
Finally, many of the 22,000 highway bridges in California are at risk of severe damage or collapse in a major earthquake (77). Any plan for earthquake hazard mitigation in a seismically active area such as California should also give high priority to the systematic retrofitting of transportation structures.