Commentaries at the San Antonio Report provide space for our community to share perspectives and offer solutions to pressing local issues. The views expressed in this commentary belong to the author alone.
Summers have become a lot hotter in San Antonio. The experts predict that this trend toward increasingly dangerous heat will continue for decades. No one has probably felt the hotter temperatures more than a transit patron at a bus stop waiting on a summer afternoon to go home from work.
Ideally, a cool bus shelter would not only provide relief for passing pedestrians but also for waiting transit passengers, even to the extent of increasing transit ridership. Commuting by transit, walking and cycling all decrease with very high temperatures and auto commuting increases adding to the greenhouse gas and other air pollutant emissions. Providing this relief is quite a challenge, however, because of the unprecedented extreme heat we are now experiencing in the summer.
To solve this problem, it is important to know how cool a bus stop needs to be. It depends on what you are used to, your physical condition, what you are wearing and how active you are (sitting, walking or doing something more strenuous). People in San Antonio might prefer to be in the shade with a temperature of less than 90 degrees. While the official San Antonio air temperature is measured at the airport in the shade, many places in San Antonio will be hotter than the official temperature, even in the shade.
The best way to measure human heat stress is the wet bulb globe temperature (WBGT). The WBGT measure was developed by the military in the 1950s in response to heat-related deaths during training. Combining air temperature, humidity, wind and heat radiation, e.g., from the sun, this measure is used today by the military, athletic organizations, the Occupational Safety and Health Administration (OSHA), the National Weather Service, and others concerned about human heat stress. A WBGT less than 78.3 degrees is a “low threat” thermal stress level.
My WBGT meter was beeping at the “high threat” level with a WBGT reading of 86.5 degrees as I stood at the unsheltered bus stop No. 87556 on the east side of San Pedro Avenue between Oblate Drive and Sprucewood Lane at 3:30 p.m. on Aug. 22. The official air temperature was 105 degrees, while the air temperature at the stop was 118 degrees. The surface of the concrete bench was 147.5 degrees, enough to cause first and second-degree burns depending on the length of exposure. The sidewalk in front of the bus stop was 137 degrees.
Some things to keep in mind about bus stops:
- the temperature at the stop depends what is around it (e.g., grass, trees, asphalt, concrete)
- the materials, coatings, color and design of the shelter are critical
- the fact that the sun moves across the sky during the day and how effectively it provides shade in the afternoon
- whether the shelter actively provides any cooling
Clearly, more than just shade is needed to protect transit users and pedestrians from the extreme heat. So, what can be done?
In Seville, Spain, a new transit shelter cools by using an underground water cistern. In Madrid, Spain, a new shelter uses evaporative cooling to lower the temperature by as much as 16 degrees. Other transit systems, like Phoenix, cool the air by misting water. The Houston Metro is experimenting with solar-powered fans, but these can be unhealthy when temperatures are 95 degrees or more without accompanying misting. A new technology called “passive radiative cooling” can reduce the temperature of horizontal surfaces by 18 degrees just with a special paint or film. Miami has air-conditioned transit shelters. Tempe has transit shelters with “green” walls with live plants.
So, our community has options. This is more than just a transit system issue. I think it is important to start learning by implementing some of these alternative bus stop concepts.
