Why Do Some Speed Limit Signs Have Flashing Yellow Lights?

There are speed limit signs of all shapes and sizes out there, but only some have flashing yellow lights. At first glance, that may sound counterintuitive, given that any flashing could distract the driver. However, capturing drivers' attention is exactly the goal of these signs. Whenever a speed limit sign has flashing yellow lights, that means there's an unexpected hazard nearby, such as road construction or heavy pedestrian traffic.

These signs temporarily override the current speed limit in the area and are usually only active during certain conditions. You'll most likely find speed limit signs with flashing yellow lights near a school. Depending on the jurisdiction, the flashing may only be active during school hours or when children are present. However, this isn't always the case, and the bottom line is that if the sign is flashing, you must obey the current speed limit or risk a fine. This is particularly true if you find yourself in a U.S. state where it's easy to get a speeding ticket.

Yellow has been standardized as a universal symbol for caution even outside of speed limits, because of science. The color sits at a medium wavelength of around 580nm in the visible spectrum. This means our eyes can perceive it more easily than most other colors (which is why school buses are yellow), with red sitting at the top of the spectrum. An Iowa State University study showed that "under normal lighting conditions, the eye is most sensitive to a yellowish-green color."

In addition, the Federal Highway Administration ran a test, showing that colorful warning signs that were colored yellow and fluorescent yellow-green were easier to spot at wider angles compared to the standard black and white signs, especially in urban areas. The same agency also published guidelines on speed limit signs with flashing yellow lights. Namely, 50 and 60 flashes per minute, and each flash should stay on for at least half to two-thirds of the full cycle.