Why are blue lights used to indicate the locations of telephones alongside subway tracks?

The use of blue lights alongside subway tracks to indicate the locations of telephones is primarily to signify emergency equipment. The blue color is a standardized marker that is easily recognizable by train operators and emergency personnel, signaling that an emergency phone is nearby. This helps ensure that in the event of an emergency, individuals can quickly locate a telephone to communicate their situation.

While visibility at night is an important element of any signaling system, the primary role of blue lights in this context is to denote the presence of emergency equipment. This visual cue acts as a safety feature, allowing for quicker response times and improved situational awareness for both the transit staff and the passengers. The differentiation from other light colors, such as green, which is commonly used for train signals, further emphasizes the specific purpose of blue lights in the emergency context.