Connecting Transportation Scenarios and Extreme Heat Exposure in Urban Areas

Increasing the portion of travelers walking, bicycling, and using public transportation to meet daily needs is a favored transportation policy goal that could lead to improvements in air quality and public health and reductions in greenhouse gas emissions in cities across the United States. But the temperatures within increasingly hot cities may pose serious risks to already vulnerable residents using these means of transportation. Although transportation-related outdoor exposures represent only a fraction of the total environmental experience of most urban dwellers, they are potentially some of the most important with respect to risks associated with extreme heat because they occur in concert with physical exertion. Because low-income individuals are less likely to own vehicles and more likely to walk, bike, or take transit, it is important to understand the potential inequities of various transportation planning scenarios, as well as provide adequate resources and information to public agencies to make non-motorized and public transportation safe under continued warming.

Despite the connection between travel behavior and heat exposure, there is virtually no prior research that quantifies its extent or examines its implications for future transportation policy. We demonstrate an approach for measuring transportation-driven exposure to dangerously high outdoor temperatures using activity-based travel demand model output and high-resolution urban climate data from the San Francisco Bay Area in the United States. Disaggregate, simulated one-day travel patterns for all Bay Area residents were joined with 1-kilometer interpolated temperature data from DayMet that was validated against observations from in situ meteorological stations. Within a geographical information system framework, Individually Experienced Temperatures (IETs) were estimated for each of the 3.6 million trips made by walk, bike, or public transit during the simulated travel day in 2010. These individual exposure time series were then aggregated to population-wide measures of Extreme Heat Degree-Minutes. Disparities in heat exposure are evident across socioeconomic, racial, and geographical dimensions, and in some cases these contrasts are amplified under different future transportation plans. Opportunities to reduce public health risks and inequities associated with high temperatures may be possible with urban design and planning strategies that target the specific places and sub-populations where this integrative approach reveals extreme heat exposure to be most severe.