Imagine taking the bus on a hot summer day. The Sun is beating down strongly, so you hurry to the bus shelter. After all, the structure’s shade should lessen the heat stress. New research, however, shows that this isn’t always the case. In fact, sometimes it makes an already unpleasant situation even worse.
Researchers from the University of Texas Health Science Center at Houston measured heat stress at bus stops in Houston between July 20 and August 7, 2023. As detailed in a study published earlier this year in the journal Transportation Research Part D: Transport and Environment, they found that, on average, trees reduced heat stress by the same amount as the best bus shelter design—and that under certain conditions, one bus shelter actually made heat stress worse. Their findings have direct implications for public health, and specifically for how experts should design future bus stops.
“Heat stress is a combination of factors, not just the temperature you see on your phone’s weather app or your car dash,” co-author Kevin Lanza said in a University of Texas Health Science Center at Houston statement. “Imagine it’s 100 degrees out. If the sun is also strong and hitting you, that continues to heat you up.” Standing in extreme heat for long periods of time puts people at risk of heat stroke, and that risk increases with high humidity, according to the statement.
The team claims that its study is the first to investigate how shelters and trees at bus stops impact heat stress via ground measurements. The team investigated trees and four different bus shelter designs at 17 Houston bus stops, collecting data from both shaded and unshaded areas, when temperatures reached 95 to 103 degrees Fahrenheit (35 to 39.4 degrees Celsius). They factored in variables such as the time of day, shelter design, and tree canopies, and recorded wet bulb globe temperature (WBGT)—a measurement of heat stress that takes into account factors such as temperature, humidity, and wind.
According to their results, the average WBGT at unshaded regions of bus stops was 92.5 degrees F. Unsurprisingly, standing in the shade generally reduced the WBGT. Tree shade decreased the WBGT by 5.9 degrees F, as did a stainless steel frame with a roof and glass walls, which was the most effective a cooling. By contrast, bus shelters with an aluminum frame, a moderately domed roof, and translucent acrylic walls lowered the WBGT by 1.6 degrees. Another similar design had a bigger and shallower dome, as well as an overhanging roof, and decreased the WBGT by 2.9 degrees F. But when the shelter itself wasn’t in the shade, the WBGT beneath it was 5.2 degrees F higher than unshaded areas.
“Because the enclosed acrylic walls with metal trapped radiation inside the shelter, it was like an oven, making it hotter than outside the shelter,” Lanza said. “Study results can serve as evidence for planting trees at bus stops for maximum cooling, and if shelters are the only option, then being strategic in design.”
Simply put, this latest study joins a host of evidence suggesting that trees in cities are almost always a good idea.
