Risk Analysis DOI: 10.1111/risa.12802 Robustness Assessment of Urban Road Network with Consideration of Multiple Hazard Events Yaoming Zhou, 1 Jiuh-Biing Sheu, 2 and Junwei Wang 1,* Robustness measures a system’s ability of being insensitive to disturbances. Previous stud- ies assessed the robustness of transportation networks to a single disturbance without con- sidering simultaneously happening multiple events. The purpose of this article is to address this problem and propose a new framework to assess the robustness of an urban transporta- tion network. The framework consists of two layers. The upper layer is to define the robust- ness index based on the impact evaluation in different scenarios obtained from the lower layer, whereas the lower layer is to evaluate the performance of each hypothetical disrupted road network given by the upper layer. The upper layer has two varieties, that is, robustness against random failure and robustness against intentional attacks. This robustness measure- ment framework is validated by application to a real-world urban road network in Hong Kong. The results show that the robustness of a transport network with consideration of mul- tiple events is quite different from and more comprehensive than that with consideration of only a single disruption. We also propose a Monte Carlo method and a heuristic algorithm to handle different scenarios with multiple hazard events, which is proved to be quite effi- cient. This methodology can also be applied to conduct risk analysis of other systems where multiple failures or disruptions exist. KEY WORDS: Heuristic algorithm; Monte Carlo; multiple hazard events; probability theory; robust- ness assessment; transportation system 1. INTRODUCTION The transportation network is one of the most critical infrastructure systems for daily operation in urbanized areas. A qualified urban system should be able to meet the requirements of complex and variable demand on the network. (34) However, the balance between supply and demand may be broken by different disturbances, ranging from random failures, such as adverse weather, road maintenance work, or traffic accident, to intentional 1 Department of Industrial and Manufacturing Systems Engineer- ing, University of Hong Kong, Hong Kong. 2 Department of Business Administration, National Taiwan Uni- versity, Taipei, Taiwan, R.O.C. ∗ Address correspondence to Junwei Wang, Department of In- dustrial and Manufacturing Systems Engineering, University of Hong Kong, Pokfulam Road, Hong Kong; jwwang@hku.hk. attacks originating from terrorist attacks or political campaigns. The event called “Occupy Central” in Hong Kong in 2015, and the July 7, 2005, London bombings are two typical examples of intentional attacks. In both events, the transportation systems were largely degraded because many critical road links were blocked or train stations were closed. As a result, social and economic activities were severely influenced. The ability of a transportation network to resist disturbances and maintain predesigned func- tionality is referred as robustness, and the method to assess the robustness of a transportation network is thus significant for transportation departments in the network design and control. The concept of robustness has been well studied in engineering. According to the IEEE the Standard computer dictionary, (1) robustness is defined as “the degree to which a system or component can function 1 0272-4332/17/0100-0001$22.00/1 C  2017 Society for Risk Analysis