A new study by a team of researchers from The University of Texas at Austin, Arizona State University and the University of Washington reveals that extreme heat significantly alters how people go about their daily lives, influencing everything from time spent at home to transportation choices.
The study, led by Arizona State University, provides detailed insights into how extreme heat affects daily activity-travel behavior and time use patterns for different socio-demographic groups. It draws on data from the American Time Use Survey and weather data from the National Oceanic and Atmospheric Administration, focusing on 11 major U.S. metropolitan areas: Atlanta, Chicago, Dallas, Houston, Los Angeles, Miami, New York, Philadelphia, Phoenix, Seattle and Washington, D.C.
“Getting out of the house, especially for social activities, is critical to our well-being as humans,” said Chandra Bhat, professor in the Cockrell School of Engineering’s Fariborz Maseeh Department of Civil, Architectural and Environmental Engineering and one of the authors of the new study. “At the same time, in many parts of the country and the world, extended heat waves caused by climate change are forcing us to stay inside, disrupting daily activity routines and our very way of life.”
The study, recently published in Transportation Research Part D, underscores the urgent need for policy action as cities worldwide grapple with rising temperatures, the researchers say.
Heat alters daily routines
According to the study, extreme heat significantly reduces the amount of time people spend outside their homes. On extremely hot days, people are more likely to stay indoors, cut back on outdoor activities, and avoid non-essential travel. For instance, the data shows a marked decrease in trips made for leisure, shopping and socializing when temperatures soar. Additionally, people shift their travel to cooler times of the day, opting for early morning or late evening trips to avoid the midday heat.
The research also shows a clear shift in transportation choices under extreme heat conditions. Car use increases, while trips made by walking, biking and public transit drop significantly. On average, public transit trips fall by nearly 50% on extreme heat days, as individuals seek relief in air-conditioned private vehicles. The authors note that this shift presents significant challenges for cities aiming to promote sustainable transportation options such as walking and public transit, especially as communities experience climate extremes with greater frequency.
Vulnerable groups face the biggest challenges
The study also found that some groups are particularly vulnerable to the adverse effects of extreme heat. Low-income individuals and those without access to cars are more likely to rely on walking or public transportation, modes that leave them exposed to dangerous temperatures. These individuals are also those with the least flexibility regarding when and where they work, thus necessitating travel even when temperatures are oppressive.
While higher-income individuals reported making significantly fewer trips on extremely hot days, lower-income individuals and those without access to a car did not show appreciable drops in daily trip-making, suggesting that they are more vulnerable and exposed to the deleterious effects of extreme heat. The research also highlights that older adults experience greater challenges in adapting their daily routines and risk experiencing social isolation on days that they shelter indoors from the heat.
“We see that extreme heat exacerbates inequities in mobility and activity-travel participation,” said Ram M. Pendyala, director and professor at ASU's School of Sustainable Engineering and the Built Environment (SSEBE) and the lead faculty researcher on the project. “Those who are already at a disadvantage, such as low-income individuals or those dependent on public transportation, face even greater risks during heat waves. This is a clear call for targeted policy interventions to protect the most vulnerable population groups.”
Policy recommendations to tackle the heat
Considering these findings, the authors propose a range of policy recommendations to help mitigate the impacts of extreme heat on communities. These include creating shaded public spaces, offering vouchers for on-demand door-to-door transportation for vulnerable populations and declaring "heat days" when people are encouraged to stay indoors, excused from traveling to the workplace, and able to access cooling centers if their homes are not adequately temperature controlled. In other words, the study calls for declaring “extreme heat” days similar to “snow” days when workplaces, businesses and schools are often closed to protect the public. Urban design improvements such as planting more trees and using heat-reflective materials on pavements can also help make cities more resilient to rising temperatures.
"Addressing the challenges associated with extreme heat is not just a matter of urban design, but of equity and public health and wellbeing," said lead author Irfan Batur, a research assistant professor in the SSEBE at ASU. "We need to take decisive action to ensure that our cities are equipped to protect all residents from the dangers of extreme heat."
A warming future requires adaptation
With extreme heat events becoming more frequent and severe due to climate change, the study’s findings are especially timely. The research highlights the critical need for cities to incorporate heat mitigation strategies into their transportation and urban planning efforts.
“We hope that our findings will inform urban planning and transportation policies to alleviate extreme heat impacts and build heat-resilient cities in the face of projected continued climate change” Bhat noted.
Other members of the research team include Victor O. Alhassan, Mikhail V. Chester and Steven E. Polzin from Arizona State University and Cynthia Chen from the University of Washington.
The collaborative effort emphasizes the importance of understanding the effects of extreme heat on everyday activities and mobility choices to inform policy solutions that can address growing climate challenges. It has been enabled through two major sponsored research initiatives, including a National Science Foundation LEAP-HI (Leading Engineering for America’s Prosperity, Health, and Infrastructure) project led by the University of Washington and two U.S. Department of Transportation University Transportation Centers) namely the TOMNET center, led by ASU, and the National Travel Behavior and Demand center, led by The University of Texas at Austin.
