Bus Shelters Aren’t Always a Cool Oasis: Study Reveals How Some Designs Can Increase Heat Stress

Imagine the sweltering heat of a summer day. The sun blazes relentlessly, baking everything it touches. Seeking refuge, you rush towards the nearest bus shelter, envisioning a brief respite from the oppressive heat. After all, shouldn’t that structure provide some much-needed shade and relief?

However, new research throws a wrench into this comforting assumption. A recent study suggests that bus shelters, in certain circumstances, might not be the cooling havens we perceive them to be. In fact, some designs could actually exacerbate the heat stress, transforming an already unpleasant situation into something even more unbearable.

Researchers from the University of Texas Health Science Center at Houston embarked on a mission to investigate the actual impact of bus shelters and trees on heat stress. They conducted meticulous measurements at bus stops throughout Houston during the peak of summer, from July 20 to August 7, 2023. The findings, published in the journal Transportation Research Part D: Transport and Environment, paint a complex picture of how these structures interact with the environment.

The study revealed a surprising truth: trees, on average, provided the same level of heat stress reduction as the most effective bus shelter design. Even more startling, under specific conditions, one particular bus shelter design actually increased heat stress compared to standing in the open sun.

These findings have profound implications for public health, particularly for those who rely on public transportation. Understanding how bus stop design and landscaping impact heat stress is crucial for creating safer and more comfortable environments for commuters.

"Heat stress is a multifaceted issue, not simply the temperature you see on your phone or car dashboard," explained co-author Kevin Lanza in a statement from the University of Texas Health Science Center at Houston. "Imagine a day where the temperature hits 100 degrees. If the sun is beating down on you relentlessly, that intense solar radiation continues to heat you up. Simply seeking shade isn’t always enough if that shade traps heat."

Prolonged exposure to extreme heat poses a significant risk of heat stroke, a potentially life-threatening condition. The researchers emphasized that this risk is further amplified by high humidity levels, common in cities like Houston.

The team’s research marks a significant step forward in understanding the microclimates of bus stops. They believe that theirs is the first study to systematically investigate the impact of shelters and trees on heat stress using direct, on-the-ground measurements.

The team meticulously examined trees and four distinct bus shelter designs at 17 bus stops scattered throughout Houston. They diligently collected data from both shaded and unshaded areas during periods when temperatures soared between 95 and 103 degrees Fahrenheit (35 to 39.4 degrees Celsius).

The researchers considered a wide array of variables in their analysis, including the time of day, the specific design of the bus shelter, and the extent of the tree canopies. They also meticulously recorded the wet bulb globe temperature (WBGT), a comprehensive measure of heat stress that accounts for temperature, humidity, wind speed, and solar radiation. This is a key element of why the study differs from someone simply reporting temperature.

The study’s results revealed a stark contrast in the effectiveness of different strategies for reducing heat stress. The average WBGT in unshaded areas of the bus stops was a concerning 92.5 degrees Fahrenheit. As expected, seeking shade generally lowered the WBGT.

Tree shade emerged as a highly effective cooling agent, decreasing the WBGT by an average of 5.9 degrees Fahrenheit. Interestingly, a bus shelter constructed with a stainless steel frame, a solid roof, and glass walls offered the same level of protection, also reducing the WBGT by 5.9 degrees Fahrenheit. This design proved to be the most effective among the shelters studied.

However, not all bus shelters provided such effective relief. Shelters featuring an aluminum frame, a moderately domed roof, and translucent acrylic walls offered a more modest reduction in WBGT, lowering it by only 1.6 degrees Fahrenheit. Another similar design, characterized by a larger, shallower dome and an overhanging roof, provided a slightly better reduction of 2.9 degrees Fahrenheit.

The most alarming finding emerged when the shelters themselves were not shaded. Under these conditions, the WBGT beneath the shelter with the aluminum frame and acrylic walls was a staggering 5.2 degrees Fahrenheit higher than in unshaded areas.

"The enclosed acrylic walls combined with the metal frame effectively trapped radiation inside the shelter, creating an oven-like effect that made it hotter than being exposed to the direct sun," explained Lanza. "Our study results provide compelling evidence for prioritizing tree planting at bus stops to maximize cooling. If shelters are the only option, then careful and strategic design is paramount."

In essence, the study underscores the importance of considering the interplay between shelter design, environmental factors, and human physiology. A poorly designed bus shelter can inadvertently transform into a heat trap, exacerbating the discomfort and health risks faced by commuters.

The findings of this study reinforce a growing body of evidence that highlights the crucial role of trees in urban environments. Trees not only provide shade but also cool the air through evapotranspiration, reducing the overall urban heat island effect. The study emphasizes that integrating trees into bus stop design should be a priority.

The study provides concrete recommendations for improving bus stop design. Architects, city planners, and public health officials can use these insights to create more comfortable and safer environments for bus riders.

The implications of this research extend far beyond Houston. Cities around the world are grappling with the effects of climate change, including rising temperatures and more frequent heat waves. Understanding how to mitigate heat stress in public spaces, such as bus stops, is essential for protecting public health and ensuring the well-being of urban populations.

This latest study adds to the mounting evidence that trees in cities are almost always a beneficial investment. As urban areas continue to grow and temperatures rise, the importance of green infrastructure, including trees and well-designed bus shelters, will only become more pronounced.