Flashing lights must be which color to effectively attract a driver's attention toward a potential hazard?

Flashing lights are most effective in attracting a driver's attention when they are yellow. The color yellow is universally recognized as a cautionary signal, indicating the need for alertness and awareness of potential hazards. This vibrant hue stands out against various backgrounds, making it easier for drivers to notice flashing lights in different conditions, such as daylight or poor weather.

While red is often associated with stop signals or emergency situations, it may not always be the best choice for indicating a general hazard, as it can signal an immediate stop rather than caution. Green is typically associated with go or safety, and blue is predominantly used for police and emergency vehicles; neither effectively conveys the need for caution like yellow does. Therefore, using yellow flashing lights is the most effective method for alerting drivers to potential dangers on the road.