Cool pavement coatings can glare: new study sets safety limits for day and night driving

Reflective “cool” pavement coatings that help cities fight summer heat may also create uncomfortable glare for drivers if their optical properties are not carefully tuned, researchers from Tongji University report in Engineering. After measuring how twelve different coated asphalt samples scatter sunlight and street-lamp light, the team proposes the first explicit thresholds for visible reflectance and specular reflection: keep daytime visible reflectance below 22% and the nighttime specular reflection coefficient under 1.5.

Raw asphalt concrete already behaves as a mixed reflector, but adding high-reflectance TiO₂-based coatings amplifies both diffuse and specular components. Without limits, the brighter surface shortens the time drivers can comfortably look at the road.

Using an automated goniophotometer that records luminance at 580 angular positions, the authors mapped how four coating colours—white, red, grey, and black—laid on three substrate porosities (7%, 17%, and 24%) reflect light when dry and after a 60 min water film was applied. White coatings raised the average luminance coefficient Q₀ to 4–5 times that of uncoated asphalt, while the specular factor S₁ dropped to only 20%–30% of the original value. Black coatings, in contrast, increased S₁ by 57%–75%, indicating stronger specular reflection despite minimal brightness gain.

Wet conditions intensified specular behaviour in every sample. Immediately after water application the specular coefficient rose 1.0–1.5-fold relative to the dry state, then gradually returned to baseline within an hour. Even a thin water film smooths micro-texture, so road lighting designers must assume the worst-case wet S₁ when they specify coatings.

To translate the lab data into real-world risk, the group modelled sun angles for Shanghai on the 2022 Equinoxes and Solstices. When the sun sat lower than 30° above the horizon—typical of winter mornings and late afternoons—white-coated pavement produced luminance values that exceeded the 2000 cd·m⁻² threshold for “strong impact” discomfort glare for up to 160 min per day. Red and grey coatings stayed below that mark but still lengthened the “slight impact” window by 80–130 min. Uncoated asphalt never reached glare levels.

Night-time simulations using 250 W high-pressure sodium lamps on a two-lane road showed that higher Q₀ always increased average pavement luminance, yet longitudinal uniformity U˪ and overall uniformity U₀ declined. The glare index TI, expressed as the percentage increase in threshold luminance needed to detect an obstacle, climbed steeply once S₁ rose above 1.5. Keeping S₁ ≤ 1.5 keeps TI below 10%, the accepted limit for safe driving. At the same time, Q₀ must stay under 0.3 to hold longitudinal uniformity above 0.7.

The authors therefore recommend that agencies specify cool coatings whose visible reflectance does not exceed 22% and whose specular reflection coefficient remains below 1.5. These numbers balance the 3–15 °C surface cooling benefit against visual comfort. Field trials that include driver-eye measurements and long-term wear tests are now under way to confirm the limits on operational roads.

The paper “Investigation on Mixed Reflection Behavior of Cool Pavement Coating and Its Impact on Safety of Road Light Environment,” is authored by Hui Li, Ning Xie, Xue Zhang, Lijun Sun, John T. Harvey, Lei Wang. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.06.014. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.