stoplights

A stoplight, also known as a traffic signal or semaphore, is a signaling device positioned at road intersections, pedestrian crossings, and other locations to control competing flows of traffic. Stoplights typically consist of three lights: red, yellow (or amber), and green. The red light indicates that traffic must stop before the intersection or crossing. The green light indicates that traffic may proceed if it is safe to do so. The yellow light serves as a warning that the signal is about to change to red, and drivers should stop if it is safe to do so.

The primary purpose of stoplights is to improve traffic safety and efficiency. By regulating the flow of vehicles and pedestrians, stoplights reduce the likelihood of collisions and minimize traffic congestion. They are essential components of modern transportation systems, enabling orderly movement through complex intersections.

The operation of stoplights can be fixed-time, meaning the signal cycles through its red, yellow, and green phases at predetermined intervals, or actuated, where sensors detect the presence of vehicles or pedestrians and adjust the signal timing accordingly. Pedestrian signals, often incorporated into stoplights, provide specific indications for people crossing the road. These usually include a walking person symbol for "walk" and an upraised hand symbol for "don't walk" or "don't start crossing." The development and widespread adoption of stoplights have significantly contributed to the predictability and safety of road travel.