Signal timing

Signal timing refers to the allocation of green, yellow, and red intervals at traffic signals to manage vehicle and pedestrian traffic efficiently. It involves determining the minimum (MIN) and maximum (MAX) green times for each movement, as well as gap, extension, or passage times that allow green lights to extend if vehicles are present. Yellow clearance indicates the duration of the yellow light, while red clearance is the all-red period between signals. Walk time denotes the pedestrian crossing interval, and flashing "Don't Walk" is calculated based on crosswalk length and pedestrian speed.

Key timing functions include cycle length—the time from one major street yellow to the next—and offset, which synchronizes signals with neighboring intersections for coordination. Coordination aims to create a smooth flow of traffic, allowing platoons of vehicles to progress through multiple green lights without stopping, enhancing traffic flow and reducing delays. However, achieving this can be challenging, especially on two-way streets.

Signal timing must balance competing factors like pedestrian safety, vehicle throughput, and varying traffic conditions. For example, longer walk times for wide crossings or slower pedestrians may increase vehicle wait times, potentially leading to accidents. Some systems adapt timings based on real-time traffic data or vary by time of day.

The Signal Timing Manual provides standardized procedures and best practices, while research continues to refine timing strategies. Notably, Mats Järlström's advocacy for adjusted yellow light timings led to changes in standards and a legal victory regarding free speech on engineering topics.