Family Handyman, Getty ImagesMost drivers don't give much thought to traffic lights other than noticing when the light changes from green to yellow to red and back again. But if you look closely, you'll notice some pretty interesting and intentional design choices.For instance, some feature a series of black lines running across the lens. Nope, those lines aren't cracks, decorative covers, shadows, or burned-out lights. They're actually louvers.Learn what those black lines, actually louvers used on Dallas Display traffic signals, are designed to do, and what drivers should do when they encounter them.What Do Dallas Display Lines on Traffic Lights Mean?Those narrow slats sit over certain traffic signal lenses and work a little like window blinds. Depending on where a driver is positioned, the louvers can block the light from view.AdvertisementAdvertisementOn traditional Dallas Displays, traffic engineers used them over the circular green and yellow lights on a separate left-turn signal. Drivers in the left-turn lane could see the signal, while someone in a neighboring through lane might only see a dark lens or black lines running across it.There was a good reason for that. If drivers going straight could clearly see a green or yellow light meant only for left-turning traffic, they could mistake it for their own signal.The Dallas Display was also part of a signal setup that addressed something traffic engineers call the "yellow trap." This can happen when a driver waiting to turn left sees their signal change and assumes oncoming traffic is stopping too, even though opposing vehicles still have a green light.As for the name, it really does come from Dallas. Traffic engineers in Dallas County, Texas, developed the Dallas Display as a way to handle intersections where drivers could make both protected and permissive left turns.AdvertisementAdvertisementSpotting the louvers doesn't require drivers to change anything they're doing. They should keep watching the signal that controls their lane. If another traffic light looks striped, dark, or strangely hard to see, it may simply be intended for drivers in a different lane.Why Aren't Dallas Display Traffic Lights Used Everywhere?The concept makes a lot of sense, right? So, why aren't they installed on all traffic lights nationwide?For one, traffic signal designs have changed. Dallas Displays were one solution for handling protected and permissive left turns, but they aren't the only option.Flashing yellow arrows have become a more familiar way to handle the same type of turn. When drivers see one, they can make the left turn after yielding to oncoming traffic and pedestrians. Research cited by the Federal Highway Administration found that drivers generally understood flashing yellow arrows better than several older protected-permissive left-turn displays.AdvertisementAdvertisementThere are drawbacks to louvers, too. While the current Manual on Uniform Traffic Control Devices still allows engineers to restrict the visibility of a signal when necessary, it notes that louvers reduce the amount of light coming from the signal. Other equipment can sometimes direct the light where it needs to go without blocking as much of it.So those black lines aren't damaged or an odd feature left over from the traffic light's manufacture. They're there so certain drivers can see the signal while others can't, which is exactly what they're supposed to do.SourcesFederal Highway Administration, "Signalized Intersections: Informational Guide"Federal Highway Administration, "Traffic Signal Timing Manual"Federal Highway Administration, "Manual on Uniform Traffic Control Devices, 11th Edition"Transportation Research Board, "Evaluation of Traffic Signal Displays for Protected/Permissive Left-Turn Control"
