We’ve all seen movie clips of stress tests: someone running on treadmill, a mass of sensors and monitors attached, a doctor watching and jotting notes on a clipboard. The speed of the treadmill steadily increases and changes in the person’s vital signs are captured.
The idea of a stress test is brilliant in its simplicity. You simulate stress on a complex system (in this case, the cardiovascular system) and watch the response, with the goal of uncovering fatal vulnerabilities. We all recognize that stress tests are an essential tool in a doctor’s toolbox when heart trouble is suspected.
Similarly, stress tests can be an essential tool in the city planner’s toolbox when vulnerability to any number of potential threats is suspected. From drought and economic downturn to more acute threats like hurricanes Harvey and Irma, or other natural disasters, testing stress levels is imperative.
A city is a lot like our heart. To run, it depends on a series of interconnected, interdependent systems that respond, react and adapt to stressors, both chronic and acute.
We’ve recently developed a software tool for stress testing the complex city system called City Simulator. Here is what’s involved when we “stress test” the cities we work with:
The key to an effective stress test for heart patients is that the test is applied to the actual complex system in question — the patient’s heart. We can’t do that with a city because our test is run over a longtime horizon — 40 years or more. But a long time horizon is essential to truly understanding the return on investment of any infrastructure strategy proposed to improve city resilience.
We don’t have to wait 40 years to see our stress test results because we develop a virtual version of the city. We develop a complete digital replica including the same number of people, road system, utilities system, zoning laws, city financial system, distribution of education and income, level of poverty, schools and churches.
Every detail that is required to capture how the city responds to stress is recreated. We also use historic data to calibrate how the city has responded to influencers in the past, like overcrowding, redevelopment, introduction of tax incentives for building in economic development zones and new commuting alternatives. (Data is cool, right?) With a virtual version of the city loaded and ready to go, we are ready to move on to simulation, the heart of stress testing.
The objective of the stress test is to find the best set of strategies for a city — strategies that will result in the greatest resilience without blowing the budget. First up is a base scenario, which in most cases is the same as the current city master plan(s). For many cities, it’s valuable just to see how resilient their current master plan makes them to various risks. We then work with stakeholders to define other scenarios, which can vary from tried-and-true methods (for example, encouraging redevelopment in the urban core) to newer technologies (like smart parking, automated vehicles and green roofs). The objective of this process is to arrive at a set of scenarios stakeholders feel are in tune with the city’s needs. Those various scenarios are then loaded into the simulator.
To really understand how different resilience solutions may work, we have to look at them under times of both business-as-usual and disasters. Moreover, we have to simulate the city in a way that incorporates the projected impacts that drivers like climate change and economic growth will have. Recent activities like cities experiencing a 500- or 1,000-year storm event, three times in three years, have real impacts on a city that wouldn’t come to light if the disasters were more widely spaced in time or were less intense. To truly understand a city’s potential resilience, we simulate all the worst-case events and measure how the city responds.
As you can imagine, the simulated model creates a lot of data. That’s why we focus on the key performance indicators (KPI) that stakeholders have already agreed are important to them when making a decision on the city plan they’d like to adopt. So, for example, we’ll measure that range from economic performance, vehicle miles traveled, carbon footprint, demographic shifts, reducing or increasing poverty and so on. Typically, we will present these metrics as a menu to stakeholders to choose from at the beginning of the exercise and measure the variations in them for each different scenario entered into the system.
The stress testing process helps identify tangible pros and cons of each scenario, the best and worst cases. It pinpoints the risk each scenario will require a city to take, and what the potential benefit will be. Ultimately, like a stress test the doctor makes you take, the City Simulator is designed to help city leaders prescribe a plan that can result in the best outcomes for the city’s long-term health and resilience.
Significant momentum is required to make meaningful changes in a city. Virtual stress testing in a way that provides understandable, tangible, and compelling evidence, can help demonstrate the value of investments and create that much needed momentum for city leaders.
City leaders are starting to take notice. Stress tests are being applied to a variety of assets in a wide variety of sectors like nuclear power plants, cyber security, terrorism, oil spills, and banking. For example, the Federal Reserve puts big banks through an annual stress test to determine if the banks can weather losses like those experienced in 2008.
In all cases, stress testing our cities represents the next frontier in using data and technology to inform challenging urban planning decisions by shedding light on the complex interactions between interconnected systems.
So much has been written lately about how cities are our nation’s beating hearts, and I agree. But those beating hearts face any number of acute and chronic stressors. We must understand how our nation’s hearts will respond to those stressors in order to choose the best course of corrective action, to help prevent the devastation Texas, Louisiana and Florida have experienced in recent days. The best way to understand and progress is to stress test.