India, Oct. 28 -- Phoenix tried a deceptively simple way to fight extreme urban heat: make its streets reflect more sunlight. The city's 2020 Cool Pavement Pilot Program coated sections of residential roads in eight neighborhoods with a lighter, more reflective seal. The goal was to reduce the amount of solar energy absorbed by asphalt and, in turn, lower temperatures across neighborhoods where summer heat can make outdoor surfaces dangerously hot.

But the experiment revealed a less obvious trade-off. The reflective pavement did make the road surface substantially cooler, yet it also sent more solar radiation back toward people nearby. For pedestrians walking directly on the treated pavement, that meant a measurable increase in radiant h...