signal timing

Signal timing is the calculated sequence and duration of green, yellow, and red phases at a traffic light to manage vehicle and pedestrian flow through an intersection. A traffic engineer sets the length of each phase, the order in which they appear, and how they coordinate with neighboring signals to move traffic efficiently and safely. Without deliberate timing, intersections become bottlenecks or collision hazards regardless of how well the roads are built.

The basic unit is cycle length, the total time from the start of one green phase to the start of the same phase again, typically ranging from 40 to 120 seconds depending on traffic volume and intersection geometry. Within a cycle, a green phase lasts long enough to clear a queue of vehicles, a yellow phase gives drivers notice to stop (usually 3 to 6 seconds), and a red phase holds cross traffic. All-red intervals of 1 to 2 seconds are inserted between incompatible movements to prevent collisions when one direction's queue is clearing and the perpendicular direction is about to start.

Signal timing varies by time of day and day of type. Morning rush hour demands a longer green on the corridor carrying commuters into town; afternoon rush demands the opposite. Weekend timing differs from weekday. Many intersections use adaptive signal control systems that detect vehicle presence via loops, cameras, or radar and adjust phase splits in real time rather than running a fixed schedule. However, fixed timing is still common in residential areas and smaller towns where demand is steady.

Offset and Coordination

When signals are spaced along a corridor, offset, the time delay between when one signal turns green and the next one does, creates a progression. Correct offset lets a platoon of vehicles travel at a steady speed without stopping; poor offset makes drivers hit every red light. Coordinated signal timing is especially valuable on arterial streets where throughput matters. The Federal Highway Administration's manual on traffic signal timing is the standard reference in North America.

Common problems include timing that favors one direction at the expense of safety for the other, cycle lengths too short to serve all approach volumes, and yellow times that are mathematically inconsistent with the approach speed and grade. Poor timing can increase delay, fuel consumption, and crash risk. A qualified traffic engineer uses volume counts, speed surveys, and simulation software like SYNCHRO or VISUM to optimize timing, then field-tests the result before full deployment.