Designing infrastructure for a changing climate remains a major challenge for engineers. In popular discourse a narrative has emerged that infrastructures are likely undersigned for the future. Weather-related hazards are directly embedded in the infrastructure design process. Yet the codes and standards that engineers use for this risk analysis have been changing for decades, sometimes increasing and other times decreasing design values. Further complicating the issue is that climate projections show increasing or decreasing intensities depending on the hazard and region. Thus, it is not clear that infrastructure is universally underdesigned. Here, analyses are developed at both regional and national scales using precipitation and roadway drainage systems to answer this question. First, it is shown that modeling uncertainty can pose challenges for using future projections to update region-specific standards. Second, the results show that depending on the historical design conditions and the direction of projections, roadway drainage infrastructure may be designed Please cite the final version of this paper: Underwood, B.S., Mascaro, G., Chester, M., Fraser, A., Lopez-Cantu, T., Samaras, C. (2020). Past and Present Design Practices and Uncertainty in Climate Projections are Challenges for Designing Infrastructure to Future Conditions. Journal of Infrastructure Systems. 26(3) 04020026. https://www.doi.org/10.1061/(ASCE)IS.1943-555X.0000567 2 appropriately in some regions while in others they are possibly underdesigned. Given these uncertainties, the authors believe that there is a need for alternative design paradigms, and these needs are discussed. Keywords: climate change; infrastructure; resilience; failure; uncertainty Introduction Infrastructure systems are the front line of defense against climate and weather-related disruptions. While infrastructure normally operate well when subjected to hazards, extreme events can push systems close to or beyond their capacities, leading to failure in one of two ways: 1) structural, where physical components of the infrastructure break down due to use beyond design expectations: e.g., Oroville dam (ASDSO 2018; Vahedifard et al. 2017) and the San Bernardino, California, Interstate 10 bridge washout; and 2) functional, where despite the structure remaining intact, operating conditions are exceeded and an infrastructure element or system cannot be used as intended: e.g., flooding of Interstate 10 in downtown Phoenix in Fall 2014 (NBC News 2014). Current engineering design uses a risk-based approach to account for hazards in the operation of infrastructure. Hazards may exist in many forms including loading, use, and climate. Risk-based approaches quantify the uncertainty of the critical factors that affect performance (either indirectly through pre-selected design safety factors or more directly using statistical uncertainty of environmental factors like precipitation or heat), then select acceptable levels of failure, and finally design the physical elements to function to the chosen risk levels. With respect to climate, current design standards are based on the analysis of historical records under the hypothesis of stationarity (USACE 1996; Olsen et al. 2015; ASCE 2018). While Please cite the final version of this paper: Underwood, B.S., Mascaro, G., Chester, M., Fraser, A., Lopez-Cantu, T., Samaras, C. (2020). Past and Present Design Practices and Uncertainty in Climate Projections are Challenges for Designing Infrastructure to Future Conditions. Journal of Infrastructure Systems. 26(3) 04020026. https://www.doi.org/10.1061/(ASCE)IS.1943-555X.0000567 3 this has been a practical and effective approach to deal with largely unpredictable events, two factors challenge the efficacy of this practice. First, federal and state infrastructure design manuals direct engineers to precipitation frequency atlases to obtain rainfall characteristics. These atlases have been updated over time as new historical climate observations become available and statistical analyses improved (USDOC 1943-1966; Hershfield 1961; Bonnin et al. 2011). As a result, the magnitude of climate variables predicted by these standards for the same level of risk may have changed over time. A recent study by Lopez-Cantu and Samaras (2018) has shown that differences among currently used United States (U.S.) precipitation estimates compared to previously used precipitation estimates obtained by stormwater engineering standards released over the period 1961-2000 are statistically significant in about 90% of the area of the 43 states where data were available. The practical implications of this finding are that (i) existing infrastructure may be over- or underdesigned according to the current standards, and (ii) the risk level of infrastructure that are designed with today’s standards may change in the future. The second factor challenging current risk-based approaches is that theoretical analyses (Trenberth 1999; Emori and Brown 2005) and an increasing number of studies based on observations (e.g., Barros and Evans 1997; Mallakpour and Villarini 2015) and climate model outputs (Milly et al. 2008, IPCC 2014) suggest that climate dynamics may evolve in a nonstationary fashion due to climate change. Some researchers have developed a theoretical background to compute climatological design factors under nonstationary conditions as a function of climate model outputs (e.g., Cheng and Kouchak 2014; Read and Vogel 2015; Salas and Obeysekera 2013). However, several sources of model, data, and scenario uncertainties still prevent the practical and standardized implementation of such methods. For example, the main sources of uncertainties of General Circulation Models (GCMs) are due to model structure (the Please cite the final version of this paper: Underwood, B.S., Mascaro, G., Chester, M., Fraser, A., Lopez-Cantu, T., Samaras, C. (2020). Past and Present Design Practices and Uncertainty in Climate Projections are Challenges for Designing Infrastructure to Future Conditions. Journal of Infrastructure Systems. 26(3) 04020026. https://www.doi.org/10.1061/(ASCE)IS.1943-555X.0000567 4 representation of the physical processes varies across models), forcings (future human activities and natural events influencing climate are unknown), and internal climate variability (climate naturally exhibits fluctuations that can be larger than changes caused by human activities) (Deser et al. 2014). These uncertainties raise two main questions about using climate projections within the traditional design process: 1) what kind of future with respect to greenhouse gases and land use should be considered? and 2) which model(s), and which output values within selected models, within any future scenario should be selected? These choices are often made by climate modelers, by determining which scenarios and models to focus model output. In addition, engineers, researchers, and other stakeholders interpreting climate projections for design process inputs also make these choices, which implicitly project risk p