41.5. Cycle time

Cycle time is the time taken by a signal to complete one full cycle of iterations. i.e. one complete rotation through all signal indications. It is denoted by the letter C.

Cycle time refers to the total duration it takes for a traffic signal to go through all its light phases—green, yellow, and red—before starting over again. This cycle is crucial in traffic signal design as it affects how long vehicles will wait at the intersection and the overall flow of traffic. When planning a signal, the designers need to calculate this time accurately to balance the needs of different traffic directions and ensure smooth transitions.

Think of a traffic signal's cycle time like the seconds in a minute. Just as the minutes of a clock go round continually, the cycle time ensures that all vehicles get a chance to move through the intersection in a timely manner, allowing for a safe and orderly flow.

The way in which the vehicles depart from an intersection when the green sign is initiated will be discussed now. Figure 41:6 illustrates a group of N vehicles at a signalised intersection, waiting for the green signal. As the signal is initiated, the time interval between two vehicles, referred to as headway crossing the curb line is noted.

Headways are the time gaps between vehicles as they start to move when a green signal is given. The first vehicle may take longer to get going due to the driver's reaction time and any acceleration needed. Subsequent vehicles can move more quickly because they start moving as soon as the vehicle in front begins to move. This concept is essential for understanding how traffic starts to flow again after stopping at a signal.

Imagine waiting at a concert. As soon as the doors open (the green signal), the first person will take a moment to gather their things and step out, which delays the whole line. However, once the first person moves, the following ones can quickly pass through since they've already prepared to go.

The first headway will be relatively longer since it includes the reaction time of the driver and the time necessary to accelerate. The second headway will be comparatively lower because the second driver can overlap his/her reaction time with that of the first driver’s. Finally, we can see that the headways level out.

The saturation headway is the average time between successive vehicles once they have fully adjusted to moving through the intersection under the green signal. This is important because it helps predict how many vehicles can clear the intersection in a given time frame. Knowing the saturation headway allows traffic engineers to calculate the saturation flow rate, which tells them how many cars can pass if the signal is green continuously.

Consider a line of cars at a drive-thru. Initially, the first car takes some time to place their order, but as the line flows, subsequent cars can move with only a short delay as they follow the car ahead. Eventually, a smooth and steady movement is achieved, much like after the initial few vehicles at a traffic signal.

There is another concept to find the amount of green time available. This is called effective green time. It is the sum of actual green time (G) plus the yellow and all red times (Y) minus the applicable lost times.

Effective green time is critical for understanding how much time is truly available for vehicles to move through an intersection. It considers the actual green light time, adds in the time given for warnings (yellow lights), and accounts for any delays that occur at the start (lost time). This calculation helps traffic engineers design signals to minimize congestion and ensure efficient traffic flow.

Think of planning a party. The time available to socialize (effective green time) is not just the hours you're awake (green light), but also includes breaks between the activities (yellow light) minus the time spent setting things up or cleaning (lost time).

The ratio of effective green time to the cycle length is defined as green ratio. We know that saturation flow rate is the number of vehicles that can be moved in one lane in one hour assuming the signal to be green always.

The capacity of a lane during a signal cycle can be determined by the formula that involves effective green time and saturation flow rate. This informs how many vehicles can safely and efficiently navigate through an intersection per hour. Engineers must consider this when designing signal timings to avoid backups and improve road efficiency.

Imagine a funnel at a carnival where people line up to get on a ride. The wider the funnel (effective green time), and the more orderly the line (saturation flow rate), the quicker everyone can get on the ride. If the funnel is small or gets clogged, people will have to wait longer than necessary.

During any green signal phase, several lanes on one or more approaches are permitted to move. One of these will have the most intense traffic. Thus it requires more time than any other lane moving at the same time.

The critical lane volume is the maximum number of vehicles that will want to use one lane in a particular phase of the signal. This lane determines how much green time should be allocated during that phase to ensure that the highest volume of traffic can clear efficiently. When planning traffic signals, identifying this lane helps ensure that traffic flows smoothly and minimizes delays for the majority.

Think of a busy restaurant where several groups are trying to enter at once. If one group has a larger table, they will need more time to be seated than another smaller group. Planning the seating to accommodate the larger group first can ease the flow and help things run smoothly.

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