on both the vulnerability of individual assets and the resilience of the system. For instance, the program prescribed a higher performance standard for 11 major toll bridges, such as the San Francisco–Oakland Bay Bridge, whose loss would cause major economic disruption and whose replacement would be extremely expensive.

Transportation planners can address the potential for future system adaptation by time windows. For instance, operational decisions will be focused on near-term changes in weather and climate conditions, such as more frequent and more extreme events (e.g., intense precipitation and flooding), with which transportation operators are already familiar. Retrofit decisions will determine the performance of assets for several decades and thus should use probabilistic climate forecasts that extend out for several decades to estimate hazards. Finally, land use and location decisions for new infrastructure may influence transportation systems for a century or more, so managers should use probabilistic climate projections for future climate conditions extending into the 22nd century. The decision framework described in Box 4-2 provides one way to incorporate such considerations.

ROBUSTNESS AND SENSITIVITY ANALYSIS

Particularly when they use multidecadal and century-scale climate projections, transportation managers should pay heed to potentially significant uncertainties in these estimates. In particular, when transportation managers use probabilistic risk assessments to compare alternative design choices or even when they conduct a screening analysis, they should be aware of choices or rankings that are especially sensitive to particular probabilistic estimates. Engineers commonly incorporate safety factors into designs or design standards to account for unforeseen events or abnormal forces on structures. Similarly, transportation managers should recognize that it may be difficult for climate change projections to distinguish a future 100-year storm from a future 500-year storm, or that estimates of the likelihood that sea level rise will exceed 1 m by 2100 may change significantly in the years ahead. To the extent that they can make location decisions and design choices that account for such uncertainties in their risk assessments, today’s transportation managers will help future stewards of their systems minimize avoidable surprises.



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