Few People Know What the Strange Scratches on Traffic Lights Are For

The intricate lines on the glass of older traffic lights may look like scratches, vandalism or damage caused by age. In fact, they were carefully designed lenses that played an important role in road safety before LED technology became widespread.

Older signals relied on a single, powerful incandescent bulb at the center. Without a special lens, the light would appear as an intense point and become difficult to see from wider angles. Road signals had to remain visible to drivers in several lanes, pedestrians at crossings and people approaching an intersection from different directions.

The patterned glass spread the light from that central source across the colored surface. Its more important function, however, was to counter the dangerous phenomenon known as a phantom signal. When low sunlight struck a traffic light from the front, it could enter the housing, reflect from the internal mirror and return toward a driver's eyes. A driver might then think that all three lights were on, or even mistake an unlit red light for an active one.

That confusion could lead to hesitation or sudden braking, potentially causing a rear-end collision. Manufacturers including Kopp Glass, Corning and Holophane developed different lens patterns. The best-known was the spiderweb pattern used on General Electric traffic lights in the 1940s. Its angular design redirected sunlight downward or to the side, reducing glare so that an unlit signal appeared black rather than illuminated.