Cool Routes aims to guide walkers through safer shade

A new project from Arizona State University is building “Cool Routes,” an online tool that calculates how hot people will feel along walking routes by simulating mean radiant temperature, factoring in shade from buildings and trees and even the time of day. It

On a hot day, the city can feel personal—like it’s aimed heat straight at your skin. Sidewalks and roads absorb the sun, then radiate that warmth back to pedestrians. Shade, suddenly, isn’t a comfort feature. It’s a safety requirement.

That’s the problem Arizona State University is trying to solve with a new project called Cool Routes. The tool is designed to calculate the heat you might experience along active mobility paths—so if your usual route turns into a furnace at street level. the map doesn’t just show you the fastest way. It shows you the coolest, shadiest option, thermally speaking.

Cool Routes, for now, is limited to the ASU Tempe campus. But the researchers say they plan to open-source the tool so any city can use it. They also imagine a future where popular map apps incorporate the same kind of heat-aware routing.

The idea comes from a basic mismatch in how people interpret weather. Your weather app can tell you the air temperature. But air temperature isn’t the whole story of whether you’ll feel miserable—or whether you could cross a dangerous line. Isaac Buo. an urban informatics scientist at ASU who co-led Cool Routes with Ariane Middel. director of the university’s SHaDE Lab. points to humidity as a major factor: “That is what makes being in a hot environment dangerous. because we see lower numbers on our phones.” He’s describing how heat can feel far worse than what a phone screen suggests—because higher humidity makes sweating less efficient at cooling the body. Add lack of shade, and the heat load on the human body increases further.

Cool Routes takes aim at the missing piece: shade. Instead of relying on a thermometer. the system calculates “mean radiant temperature.” In practical terms. it accounts for where the shade is coming from—buildings. trees. and the geometry of the street can all matter. The researchers can do this because the tool uses high-resolution urban mapping built from United States Geological Survey lidar technology. which has been used to map landscapes in extreme detail.

The simulation also depends on time—because the city’s shade isn’t static. Walking at 8 a.m. is different from walking at noon. not only because temperatures rise through the day. but because the sun’s angle changes where shadows fall. Buo describes a specific example at high noon: skyscrapers might not provide much shade when the sun is directly overhead. while trees can still offer cooling because they create an overhanging canopy. That kind of detail is why heat exposure can vary dramatically not just from neighborhood to neighborhood. but even block to block.

To model that variation. Cool Routes factors in heat coming from six directions at any given point and time: from north. south. east. and west. and from above and below. The team also didn’t stop at simulation. They ground-truthed the calculations using a “mobile human-biometeorological cart” called MaRTy. where the MRT in its name refers to mean radiant temperature. MaRTy was rolled along courses suggested by Cool Routes on typical hot summer days.

That mix—high-resolution mapping. weather data. time-aware thermal simulation. and field grounding—pushes the project beyond a simple “avoid the hottest streets” concept. It’s built to help people choose a path that reduces heat exposure while they’re moving. But the potential reach extends beyond individual walkers.

Cities could use the platform to decide where tree planting and parks are most urgently needed. If there’s a popular route people routinely follow—from a subway station to a financial district. for example—adding vegetation along that corridor could reduce temperatures by several degrees. Trees. the researchers emphasize. are also a multi-purpose tool: in addition to cooling. green patches provide habitat for animals and absorb stormwater. reducing the risk of flooding.

Even public transportation could benefit. While Cool Routes is meant to encourage walking, it could also help transit systems by identifying where bus stops need shade so people waiting outside aren’t exposed to harsh conditions.

For now, Cool Routes is early, limited, and still being tested. But the researchers are already thinking about how this kind of thermal routing could enter everyday life through mainstream map software. Buo says that if you’re willing to make a detour of. say. two extra minutes. the tool can guide you through a route that is well-shaded.

In a city where heat can feel like a trap, that’s the promise: a route plan that doesn’t just get you there—it keeps you safer on the way.

urban heat island shade mean radiant temperature Cool Routes Arizona State University SHaDE Lab urban informatics active mobility MaRTy lidar tree planting public transportation shade