Measuring Transportation System Resilience: Response of Rail Transit to Weather Disruptions

Major disruptions of transportation systems due to severe weather events such as hurricanes, storm surges, and flooding are becoming more common. A logical response is to make those systems better able to withstand and rebound from large-scale weather assaults. Resilience is the term commonly used to describe this property of transportation systems, and while there are numerous definitions of resilience, there is no commonly used metric to characterize it. This paper reviews the concept of resilience in different fields and offers a definition that encompasses three common strategies for making transportation systems more resilient: hardening, redundancy, and elasticity. The paper offers a graphical and analytic metric for assessing the resilience of scheduled transportation services in the face of weather disruptions, focusing on heavy rail transit (HRT) systems. The concept of planned service days lost to weather disruptions is suggested as a useful aggregate measure of system resilience that is normalized for system scale. The measure is applied to data from the New York Metropolitan Transportation Authority rail transit system to illustrate the effects of Hurricanes Irene and Sandy, as well as a major snowstorm that hit New York City, and to suggest the ways in which the proposed measures can be used for ex post evaluation of response to weather events, as well as to support system management decisions. A comparative application of the measure for New York HRT and New Jersey Transit (NJT) commuter rail illustrates the use of this normalized measure across two different transit services.