power, and coupled modeling frameworks, several limitations still exist<sup>33</sup> . Information on globally available discharge estimates, specifically, the estimation of high flows in ungauged catchments remains one of the most fundamental challenges in catchment hydrology<sup>34,35</sup> . In addition, the quality of digital elevation models, the accuracy of boundary conditions used to force inundation models, along with inadequate knowledge of river morphology have hindered the development of high-quality global flood models<sup>36,37</sup> . Second, many studies use administrative or self-defined boundaries to extract spatially embedded infrastructure networks in local and regional studies<sup>38–40</sup> . However, due to the misalignment between functional network structure<sup>41</sup> and spatial network structure<sup>42</sup> , the aforementioned approach often cuts through critical road segments leaving out network components that many residents rely on for daily commuting. Third, the emergent risk that arises from indirect, transboundary, and system-level flood disruptions is difficult to quantify and therefore rarely investigated in past studies<sup>43</sup> . The potential indirect impacts induced by floods span the social, environmental, and economic spheres, vary across different cultural contexts, and contain uncertainties surrounding the behavioral responses of individuals and communities<sup>19</sup> . More often than not, these impacts extend beyond the context of the hazard or location, reaching interdependent or interconnected networks.

In this study, we tackle the three challenges mentioned above and evaluate both direct flood hazard exposure and indirect travel disruptions of pluvial and fluvial floods on road networks in 2,564 populated settlement clusters spanning 177 countries and regions<sup>2</sup> around the world. We spatially overlay high-resolution flood inundation maps for ten flood return periods: 5-year, 10-year, 20-year, 25-year, 50-year, 75-year, 100-year, 250-year, 500-year, and 1000-year (based on Fathom global flood models at 3 arc-seconds spatial resolution, equivalent to about 90 × 90 meters at the equator<sup>3</sup> ) with worldwide road networks extracted from Open Street Map (OSM) to better understand local hazard exposure. For each return period, we simulate travel trajectories between potential origins and destinations (generated with a probability-based spatial sampling approach) within each settlement cluster and record trip details such as travel time, distance and trajectory. Our study fills a gap in the current literature by providing the first global evaluation of both direct (impacts due to the physical contact with floodwater) and indirect (impacts not due to physical contact or that occur outside the inundated area in space or time such as travel disruptions and cascading failures in interconnected or interdependent infrastructures) flood hazard impact on road transportation networks.

# **Literature Review**

Floods are the predominant cause of weather-related disruptions to the transport sector<sup>44</sup> . Specifically, floods reduce transportation network capacity, either directly through physical destruction rendering roads unusable or through flood water accumulation on the road surface rendering the road impassable<sup>2</sup> . These local network disruptions often cause increases in travel demand on distant road segments leading to severe traffic congestion that could later propagate to regional scales<sup>25,45,46</sup> . In the following sections, we present a review of existing literature on understanding direct flood hazard exposure and vulnerability and resilience of transportation networks to floods.

## **Understanding exposure**

> 2 Here we used the World Bank-approved administrative boundaries (Admin 0) as reference. In sum, there are 251 countries and regions worldwide and the settlement clusters identified in our study spatially intersect with 177. 3 Fathom uses MERIT-DEM and MERIT-Hydro.

3