Desperate for shade on your walk? There’s (almost) an app for that.

On a scorching 104°F afternoon in Tempe, Arizona, Isaac Buo wheels a strange contraption across the sunbaked campus of Arizona State University—a six-wheeled cart bristling with sensors, affectionately named MaRTy. It’s not a sci-fi prop; it’s science in action, measuring the real heat humans feel as they walk under the desert sun. This is the ground truth behind Cool Routes, a pioneering tool Buo and urban climate expert Ariane Middel have developed to map the coolest, shadiest walking paths on ASU’s Tempe campus. In a city where summer sidewalks can hit 160°F, and where the urban heat island effect turns streets into ovens, Cool Routes offers something radical: a way to walk safely through the heat.

The idea is simple but revolutionary. While every major map app can guide you from point A to B, none tell you whether that route will fry you in the process. Air temperature alone doesn’t capture how hot it feels—radiant heat from concrete, lack of shade, and sun angle all play critical roles. Cool Routes calculates “mean radiant temperature,” factoring in shade from trees and buildings using high-resolution lidar data from the U.S. Geological Survey. It simulates thermal conditions down to the block, even the minute, accounting for heat coming from six directions—north, south, east, west, above, and below. A walk at 8 a.m. under east-facing building shadows is not the same as at noon, when only tree canopies offer relief.

The team validated their models by rolling MaRTy along predicted paths on 115°F days, confirming that shaded routes can reduce heat exposure by up to 50%. One key insight: trees outperform buildings when it comes to consistent shade. Skyscrapers cast narrow shadows in midday sun, but leafy canopies create a cooling umbrella all day. That’s vital information for city planners. Cool Routes isn’t just for pedestrians—it’s a planning tool. Cities could use it to decide where to plant trees, build parks, or shade bus stops along high-traffic corridors. A detour of just two minutes, Buo says, can lead to a route that’s dramatically cooler and safer.

Now, the ASU team is open-sourcing the platform, inviting cities worldwide to adapt it. The long-term vision? A future where Google Maps or Apple Maps don’t just show the fastest route, but the coolest one. In an era of rising temperatures and urban densification, tools like Cool Routes don’t just make walks more comfortable—they make them possible. And as more cities face the reality of extreme heat, the simplest solution might be the oldest: a little shade, thoughtfully placed.