The key to understanding this issue, however, is human physiology. The eyes have multiple kinds of receptor cells, cones and rods, which work best, respectively, in daylight and at night. Cones are more sensitive to yellow hues, rods to blue light. Thus, at night, yellow lightslike common sodium-based streetlightsmay be technically as bright as bluer lights, but they won’t be perceived that way. “It’s almost as if the lighting industry forgot the science behind light,” says Bullough.
Therefore, a town installing new lights can substitute the bluer lights for the more common yellower lights, Bullough says, and in doing so, “reduce the light level, save energy, and get the same visual effects. Or you can use the same energy and increase the visibility. You can get more for less, literally.” You also may save lives. A recent study conducted by the LRC for the National Academy of Sciences supports the conclusion that street lighting, properly located, can reduce traffic fatalities.
One of Bullough’s first transportation studies involved determining the best headlights for snowplows in New York state, where plow drivers were filing complaints about their equipment and arguing that yellow light “cuts through the snow better.” In terms of physics, this is wrong: Yellow light does not “cut through” snow more than any other color.
But snowplows are usually used in heavy snow, rain, or sleet, so a lot of light bounces back at drivers. In low-light conditions, this means blue headlights, being more noticeable, would also create a more noticeable bounce-back effect. Yellow light, theoretically less effective in those conditions, would bounce back less and “seemed to reduce visual fatigue,” says Bullough. This way, he notes, the snowplow drivers “were right all along. They had the explanation wrong, but they were recommending the right solution.”
The lesson, he says, is that all light exists in a particular context and its effectiveness depends on how it is used. Recognizing this need for adaptability makes autos another candidate for LED use. LED headlights could be modular and adaptable. You might have “no more low beams and high beams,” says Bullough, but instead have sensitive LEDs adjusting to road and traffic conditions by changing their brightness and directionality.