Why Do Some Traffic Lights Have Yellow Borders? Here's What They're For

Traffic lights are one of those pieces of street furniture you probably don't think about all that much until you're stuck waiting at them. They all serve the same core purpose of letting vehicles know when it's safe to move ahead or not, but there's more variation among them than you might expect. Some traffic lights have two red arrows, others have a blinking green light, and one traffic light in New York even has upside down green and red lights thanks to protests by resident Irish immigrants.

You might also notice that some traffic lights are surrounded by yellow borders that usually stretch between 1-3 inches around the edge of the signal. These reflective yellow borders are installed to boost their visibility at night, making them more instantly recognizable by tired or distracted drivers. They also help drivers with color vision deficiencies spot them quicker. As a bonus, a reflective border will remain visible even when the lights are inoperative due to a power outage, which helps warn all drivers that there's an intersection ahead.

Installing a reflective border is one of the cheapest ways to boost the visibility of a traffic light, but research has shown that it makes a notable difference to road safety. In fact, one 2005 study found that intersections where traffic lights had reflective borders installed saw a 15% decrease in crashes.

Even a traffic light that doesn't have a noticeable visual difference like a reflective yellow border might still be different to a light that's just down the road. Another tactic employed by transport agencies to boost road safety is adjusting the amount of time each traffic light's yellow light phase lasts.

The goal is to give drivers enough time to recognize the yellow light and slow down without making it last so long that drivers start to treat it as an extension of the green light. Too long or too short and the risk of drivers running red lights increases, which can lead to a higher rate of crashes. There's no one set ideal period of time for a yellow light to last, since it depends on a range of factors like the overall layout of the intersection, the speed of the surrounding roads, and the speed that drivers will need to slow down to in order to make their turn safely.

The latest traffic lights can use automated systems to monitor and adjust the yellow light phase in real time. It's a far cry from the early, simplistic traffic lights that were used before modern computers were invented.