A pilot study exploring the effect of vehicular waste heat on personal heat exposure in a semi-arid environment

Understanding the causes of heat in various microclimates in cities is vital to improving human thermal comfort and health in outdoor spaces. This pilot study uses an experimental design to evaluate microclimate heat risk – including ambient air temperature (TA) and wet bulb globe temperature (WBGT) – and approximate personal heat exposure of people 1.5-3.0 meters (5–10 feet) away from idling vehicles. These measurements were taken at a University of Arizona covered parking garage in June 2022 to investigate personal heat exposure attributable to vehicular waste heat while also minimizing the effect of solar radiant heat and wind. We used Kestrel 5400 devices to document the waste heat effects of a fleet of six identical gasoline-powered vehicles on the surrounding microclimate by collecting TA, wind velocity, and WBGT. When compared to the control, as well as comparing a period when engines were not idling, we found a strong correlation between vehicle presence, TA, and WBGT. Specifically, ordinary least-mean square (OLS) modeling shows an additional TA per minute increase of 0.006°C (a 25% increase) per minute when the vehicles are on and idling versus 0.024°C per minute increase that occurs from expected background morning temperatures when the vehicle effect is removed. Until we transition to an electric fleet and increase use of alternative modes of transportation, these findings can help inform how transportation professionals design the built environment and manage traffic and transit during summer months to prevent excessive heat exposure for pedestrians, cyclists, transit riders, and other individuals near idling vehicles.