As Massachusetts endures its first heat wave of the summer, research highlights something many locals have intuitively felt: hot and humid weather makes city walks seem longer, prompting residents to consider driving short distances rather than walking or using public transport.
In November, Rounaq Basu, then a postdoctoral researcher at MIT, published a study titled “Hot and bothered: Exploring the effect of heat on pedestrian route choice behavior and accessibility” in the urban planning journal Cities.
Basu’s passion for hiking throughout the year led him to contemplate the effects of heat on pedestrian experiences, stating, “I love hiking throughout the year, but summer is when I take a break because it gets too hot for me. So I started thinking about how to incorporate heat into the experience of the pedestrian experience.”
His previous work revolved around how infrastructure influences travel behavior. This time, he wanted to explore how temperature affects route choices during summer, focusing on a key measurement known as the Universal Thermal Climate Index (UTCI).
The UTCI differs from mere temperature or heat index readings; it combines factors like wind speed and radiant temperature, which can change significantly based on one’s surroundings, such as shade and materials that absorb heat like asphalt.
Basu explained, “The heat index or the temperature are not as appropriate as how people perceive heat when they’re walking. If it’s 85 and there’s a nice breeze and you’re walking on Commonwealth Ave. through the trees, that feels pleasant, but the same weather conditions can feel oppressive if you’re in the middle of a big asphalt parking lot.”
Utilizing anonymized walking trip data from a fitness app, Basu and his team discovered that pedestrians were inclined to choose a longer route—averaging 81 meters (roughly a minute’s walk)—to avoid an increase in UTCI by just 1 degree Celsius.
“What that means is, if I am walking along a street, and there’s a 1 degree increase in perceived temperature, that block is going to feel like a longer walk, 81 meters longer, because it’s hotter,” Basu clarified.
The study further revealed that this effect intensified with rising temperatures, noting potential for an “exponential increase” in perceived walking distance during extreme heat.
Basu also highlighted that the implications extend beyond pedestrian choices, influencing how competitive public transportation becomes compared to car travel.
“400 meters (about a quarter-mile) is generally considered the reasonable distance to walk to a transit stop. But if you don’t take heat into account, we might be grossly overestimating accessibility. If there’s no shade and lots of pavement around, that 400-meter trip might be too much for people to walk on a hot day,” he remarked.
Ironically, opting for alternatives like driving or staying indoors with air conditioning contributes to the urban heat effect.
Additionally, not every individual has the choice to avoid walking in the heat. In Boston, non-white households tend to rely more heavily on public transportation and often reside in neighborhoods with less greenery and fewer shaded areas.
Basu acknowledged, “Heat isn’t always a factor for every single trip people make. There are certain trips that people are making no matter what the heat is like.”
As the summer progresses and temperatures rise, understanding the behavioral impact of heat on urban mobility becomes crucial for improving accessibility and comfort in cities like Boston.
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