# **Introduction**

Road transportation networks are lifelines for cities around the world, providing people with access to markets, jobs, and critical services, including emergency medical services, primary health care and fuel<sup>1,2</sup> . However, climate change, rapid urbanization, coupled with increasing interdependencies among various infrastructure systems are putting road transportation networks under increasing pressure<sup>2–5</sup> . An increasing number of studies document the growing risk and vulnerability of these networks to climate-changeinduced extreme weather events such as floods<sup>6–10</sup> , wildfires<sup>11–13</sup> , and earthquakes<sup>14–16</sup> . Among them, floods, especially as a result of intense precipitation, are the predominant cause of weather-related disruption in the transportation sector<sup>7</sup> . Such events are particularly acute on urban road networks owing to the high proportion of impermeable surfaces that prevents fast infiltration and reduces flow resistance<sup>17</sup> . Roads naturally become preferential paths for stormwater runoff. Such floods compromise the vital functionality of road transportation networks, either directly through flood water accumulation or indirectly through physical destruction resulting in decreases in local network capacity.

According to the Intergovernmental Panel on Climate Change (IPCC), the frequency of climate-changeinduced extreme precipitation events is expected to increase globally<sup>18–21</sup> . Between 1900 and 2021, a total of 5,501 reported flood disasters had been recorded around the world, affecting more than three billion people (48.4% of the total number of people affected by all weather-related disasters) and claiming more than six million lives<sup>1</sup> . In the past 30 years, the world has witnessed an alarming increase in the number of reported flood disasters, mainly driven by steady population growth and economic activities in flood-prone areas<sup>23,24</sup> . In many developing countries, the challenges associated with transport disruptions due to floods are aggravated by a lack of systematic urban planning, adequate flood control infrastructure (e.g., drainage systems), regular maintenance, technical capacity, and financial resources<sup>25</sup> . Without proactive flood risk management planning and adaptation, risks could continue to intensify as the absolute flood damage may increase up to a factor of 20 by the end of century<sup>26</sup> . When taking network complexity and cross-sector interconnectivity into consideration, such impacts will likely cascade through the networks causing systemlevel failures, severe delays, and prolonged socioeconomic damages.

To date, many studies have investigated the impact of floods on road networks in terms of hazard exposure to historical or modeled flood events<sup>27,28</sup> , travel delay as a result of flood disruption<sup>19,29</sup> , economic cost estimation<sup>2,30</sup> , and road network resilience evaluation<sup>31,32</sup> at local and regional scales. However, several challenges and limitations have so far prevented an analysis at a global level. First, the lack of publicly accessible high-resolution global flood maps capable of capturing flood events of varying intensities hinders a worldwide evaluation of flood hazard impacts on road transportation networks. Although the development of global flood models has been facilitated by advances in satellite data, numerical algorithms, computing

> 1 Historical flood records are extracted from the Emergency Events Database (EM-DAT22). Four flood hazard types are included: coastal flood, flash flood, ice jam flood and riverine flood. It should be noted that the number of deaths and the number of affected people are both conservative estimations. Three entry criteria need to be fulfilled to be entered into the database: ten or more deaths, 100 or more people affected/injured/homeless, and declaration by the country of a state of emergency and/or an appeal for international assistance. For more information refer to EM-DAT data entry guidelines at https://public.emdat.be/about. To calculate the percentage of people that were affected by floods over all weather related disasters from 1900 to 2021, we extracted the number of people affected by four types of floods: 3,879,727,358 and divide by the number of people that were affected by meteorological (extreme temperature, fog, storm), hydrological (wave action, landslide, flood), and climatological disasters (wildfire, glacial lake outburst, drought): 8,015,008,050.

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