In Phoenix, the difference between a scorching walk and a bearable one can be measured in the shade of a single tree. Arizona State University researchers have created Cool Routes, a web-based navigation tool that steers pedestrians toward the shadiest, coolest paths across a city—not just the shortest ones.
Developed by Ariane Middel and colleagues in The SHaDE Lab at ASU, Cool Routes uses hourly weather forecasts and street-level data on buildings and trees to calculate mean radiant temperature, a measure of total heat load including reflected heat from pavement and building facades. In full Phoenix sun, mean radiant temperature can exceed 150 degrees Fahrenheit; in shade, it can drop below 100 degrees. That 50-degree swing represents the real difference between a walk that feels manageable and one that feels dangerous.
"It's more comprehensive than just using air temperature or land surface temperature," said Middel, an associate professor in The GAME School at ASU. Cool Routes is believed to be the first navigation app to use real-time heat exposure data, updating itself with hourly meteorological forecasts to deliver the most current information whenever a user makes a request. Waqar Hassan Khan, a Ph.D. student in computer science, built the backend programming to calculate heat load based on surrounding buildings, trees, and weather conditions for the appropriate hour.
Testing on ASU's Tempe campus over 12 days across different seasons proved the tool's reliability. Cool Routes identified cooler alternatives for more than 70% of trips, even during morning and evening periods when heat differences are typically minimal. Crucially, these alternate routes didn't demand long detours; added distances ranged from about 25 feet to less than a city block. On average, alternate routes lowered experienced heat load by about 4.5 degrees—a meaningful reduction for pedestrians navigating extreme heat.
The researchers validated their predictions using MaRTy, a rolling instrument station that measures human thermal exposure and meteorological data. During record-breaking heat waves, postdoctoral researcher Isaac Buo and his team traversed suggested routes, confirming that Cool Routes predictions showed strong agreement with direct measurements. "When air temperatures exceed 115°F, the physical reality of the urban overheating effect is visceral," Buo said. "You can feel the intense longwave radiation radiating off the asphalt and building facades like an open oven."
Beyond individual navigation, the tool opens possibilities for urban planning. City officials could use Cool Routes data to identify busy walking corridors exposed to high heat and prioritize interventions like street tree placement or shade structures. Planners could simulate "what-if" scenarios—adding tree canopy or shading structures—to assess how these actions change pedestrian heat load and calculate return on investment for green infrastructure.
Cool Routes currently covers ASU's Tempe campus, but it operates as an open-source framework designed for deployment in other cities to support walking, biking, and public transit use. A mobile app is in development as Fletcher Emmott's honors thesis project, aiming to make the tool more accessible and user-friendly. Scaling to larger areas will require additional computing power; the underlying system must simulate thermal exposure, store massive datasets, and generate thermally safe routes in seconds rather than hours. The work has been published in the journal Building and Environment by Middel, Khan, Buo, and co-authors Evan Crabtree, Fletcher Emmott, and Devbrat Hariyani.