A traffic signal timing plan is a set of numbers and diagrams that tells a signal controller exactly when to turn each light green, yellow, and red. At first glance it looks like a dense spreadsheet, but every value on the sheet maps to something you can observe at the intersection: how long the left-turn arrow stays green, how much time pedestrians get to cross, and how neighboring signals coordinate to create a “green wave.” Once you understand a few core concepts, the rest falls into place.
Phases: The Building Blocks
The most fundamental unit on any timing plan is the phase. A phase is a single movement, or group of compatible movements, that receives a green light at the same time. A standard four-way intersection with dedicated left-turn arrows typically uses eight phases, following a numbering system established by the National Electrical Manufacturers Association (NEMA).
The convention is straightforward: even-numbered phases (2, 4, 6, 8) are through movements, and odd-numbered phases (1, 3, 5, 7) are left turns. So if you see “Phase 2” on a timing sheet, that’s a through movement on one of the main approaches. “Phase 5” would be a protected left turn on the opposing leg. Right turns usually don’t get their own phase because they can move safely at the same time as the adjacent through movement. A westbound right turn, for example, shares the same phase as the westbound through traffic.
Not every intersection uses all eight phases. A simple T-intersection or one without protected left turns might only need three or four. The timing plan will list only the active phases, so the first thing to do when reading a plan is identify which phases are present and match each one to a physical movement at the intersection.
Cycle Length, Splits, and Intervals
Three numbers define the big picture of any timing plan: cycle length, splits, and intervals.
Cycle length is the total time it takes for the signal to serve every phase once and return to the starting point. A typical cycle length might be 90 seconds at a moderately busy intersection or 120 seconds or more on a high-volume arterial. You’ll see this as a single number at the top of the timing sheet, usually in seconds.
Splits tell you what percentage (or how many seconds) of that cycle each phase gets. If the cycle is 100 seconds and Phase 2 has a 40% split, that phase gets 40 seconds total. That total includes not just the green time but also the yellow and all-red clearance intervals that follow it. When you look at the split column on a timing plan, remember that the green time a driver actually sees is shorter than the split value because a few seconds are consumed by yellow and red clearance.
Intervals are the individual color displays within a phase. Every phase contains at least three intervals: green, yellow (also called “change”), and all-red (“clearance”). The yellow interval is typically 3 to 5 seconds depending on approach speed. The all-red is usually 1 to 2 seconds, giving vehicles time to clear the intersection before conflicting traffic gets a green. On the timing sheet, you’ll often see columns labeled something like “Min Green,” “Yellow,” and “All Red” for each phase.
The Ring-and-Barrier Diagram
Most timing plans include a ring-and-barrier diagram, which is the visual map of how phases relate to each other. It looks like a grid divided into rows (rings) and columns separated by vertical lines (barriers). This diagram answers two critical questions: which phases can run at the same time, and which phases must end before others can begin.
A standard eight-phase intersection has two rings. Ring 1 might contain phases 1, 2, 3, and 4. Ring 2 contains phases 5, 6, 7, and 8. Phases in different rings but on the same side of a barrier can run simultaneously. For example, Phase 1 (a northbound left turn) and Phase 5 (a southbound left turn) can both be green at once because they don’t conflict with each other.
The barrier is the vertical dividing line that separates the north-south phases from the east-west phases. All phases on one side of the barrier must finish before any phase on the other side can start. This prevents conflicting movements, like northbound through traffic and eastbound through traffic, from having green at the same time. When reading the diagram, move left to right within each ring to follow the sequence, and look across rings to see what runs concurrently.
Pedestrian Timing Parameters
Pedestrian timing appears on the plan as two values for each phase that serves a crosswalk: Walk and Flashing Don’t Walk (sometimes labeled “Ped Clear” or “FDW”).
The Walk interval is the steady walking figure displayed to pedestrians. It typically lasts 4 to 7 seconds, just long enough for someone to step off the curb and begin crossing. The Flashing Don’t Walk interval is the countdown period that follows. Its duration is calculated by dividing the crossing distance (curb to curb, in feet) by an assumed walking speed, usually 3.5 feet per second. A 50-foot crosswalk, for example, needs about 14 to 15 seconds of flashing clearance time.
These pedestrian values directly constrain the minimum green time for the associated vehicle phase. If an intersection has no pedestrian signals or push buttons, the controller still has to hold the green long enough to equal the walk plus clearance time. So when you see a phase with a surprisingly long minimum green, pedestrian clearance requirements are often the reason.
Actuated Settings: Min Green, Max Green, and Gap
Many intersections don’t simply rotate through fixed green times. They use detectors (usually loops cut into the pavement or cameras mounted on the signal arm) to adjust timing in real time. The timing plan for an actuated signal includes a few additional parameters.
Minimum green is the shortest green time a phase will display, even if only a single vehicle triggers it. This ensures drivers have enough time to react and begin moving. It also has to be at least as long as the pedestrian walk-plus-clearance time when no separate pedestrian signal exists.
Maximum green is the ceiling. No matter how heavy traffic is on one approach, the controller won’t hold that phase green longer than this value. It protects side streets and cross traffic from waiting indefinitely.
Passage time (also called gap or vehicle extension) is the amount of time the controller waits after the last detected vehicle before ending the phase. If another car crosses the detector within the passage time window, the green extends. If no car shows up within the gap, the phase “gaps out” and ends. This is how the signal responds to real-time demand: light traffic gaps out quickly and gives time back to other phases, while heavy traffic can use up to the maximum green.
On the timing sheet, you’ll typically see these three values listed per phase. If a phase shows a minimum green of 10 seconds, a maximum of 45 seconds, and a passage time of 3 seconds, that phase can serve anywhere from 10 to 45 seconds of green depending on how many vehicles the detector picks up.
Offset and Signal Coordination
When multiple intersections along a corridor share the same cycle length, they can be coordinated so that a driver traveling at the target speed hits a series of green lights. The timing plan achieves this through a value called offset.
Offset is the time difference between a reference point in the cycle at one intersection and the same reference point at the next intersection. The reference point is usually the start of the main street green phase. If Intersection A has an offset of 0 seconds and Intersection B, half a mile down the road, has an offset of 20 seconds, then Intersection B’s main street green begins 20 seconds after Intersection A’s. That delay is calculated to match the travel time between the two signals at the desired speed, creating the progression drivers experience as a “green wave.”
On a coordination timing sheet, you’ll see a column for offset alongside the cycle length and splits. Some plans list offsets relative to a master intersection (offset of zero), while others reference the system clock. Either way, the concept is the same: offset synchronizes the start of green across intersections so platoons of vehicles arrive during the green window.
Putting It All Together
When you first pick up a timing plan, work through it in this order. Start with the cycle length to understand the overall time frame. Next, identify each phase and match it to a physical movement using the NEMA convention (odd for left turns, even for through movements) and any intersection diagram included with the plan. Then look at the splits or green times for each phase to see how the cycle is divided up.
Check the yellow and all-red clearance values to account for the transition time between phases. Review pedestrian parameters if crosswalks are present. If the signal is actuated, note the minimum green, maximum green, and passage time for each phase to understand the range of possible green durations. Finally, if the plan covers a coordinated corridor, look at the offset value to see how this signal’s green aligns with its neighbors.
One practical tip: timing plans often come in multiple versions for different times of day. A “Peak AM” plan might have a 120-second cycle with heavy splits favoring the commute direction, while an “Off-Peak” plan might drop to 80 seconds with more balanced splits. A “Free” or “uncoordinated” plan means the signal is running fully actuated with no fixed cycle length. Always check which plan you’re reading before drawing conclusions about how the intersection operates.