Appendix G 3.1 Vehicle Phase Intervals
The running part of the phase corresponds to the period when the green signal is displayed. It is the period between the phase start and the phase change points. The clearance part of the phase corresponds to the period when the yellow and all-red signals are displayed. It is the period between the phase change point (the end of running intervals) and the beginning of the green display for the next phase (end of phase). Green time is the duration of running intervals, and the intergreen time is the duration of yellow time and all-red time.
Different signal indications can be displayed to different movements using the phase during the late start and early cut-off green intervals. The late start interval is used to delay the introduction of the green signal to some movements in the phase. The durations of these intervals are determined by the late start and early cut-off green settings (Appendices G.4.1 and G.4.5).
The two minimum green intervals used for safety reasons are the basic minimum green interval and variable initial green interval (Appendix A). With stop‑line detectors, a basic minimum green setting determines the minimum green time allocated to a movement (Appendix A). The variable initial green interval is used with advanced detectors to provide additional minimum green time to discharge a queue of vehicles stored between the stop line and detectors during the red period. The duration of this interval varies in response to the number of actuations of the advance detectors and is determined according to the values of the vehicle increment and maximum initial green settings (Appendix A).
The controller cannot enter the extension green interval until a demand for another phase is registered. The rest interval is an untimed interval after the minimum green time expires, during which the controller rests until a demand for another phase is registered as shown in Figure G 1. The rest interval is skipped if a demand is registered for other phases before the end of the minimum green time.
The extension green interval is of variable length, and under isolated operation, its duration is determined by extension settings, namely the gap setting (Appendix G.4.4), headway and waste settings (Appendix G.4.7), and maximum extension green setting (Appendix G.4.3). In the case of parallel vehicle and pedestrian movements at intersections, the pedestrian walk and Clearance 1 intervals can hold the extension green interval (Appendices G.3.2 and G.5). The gap, headway and waste settings are used as ‘space’ (non-occupancy) time values as measured by presence detection.
The gap timing logic operates from the start of the green period to enable a green termination at the end of the minimum green period or the rest interval. It operates for the whole of the running part of the phase including the rest interval. The gap timer is loaded with the gap setting when a detector actuation occurs. With presence detection, the gap timer starts decrementing (from the initial value of the gap setting) when detector actuation ceases.
If the gap timer reaches zero before the next detector actuation, the timer is said to have timed out (or ‘gapped out’). When this occurs during the extension green interval, the green period is terminated (subject to parallel pedestrian movement timing constraints). This point during the phase is called the phase change time. This type of extension green termination will be called a gap change. The phase change process starts with the yellow signal display unless there is an early cut-off green interval in which case the early cut-off yellow interval starts (Appendix G.4.5).
The current Australian control method employs headway and waste settings as additional extension settings. The headway-waste control method aims to terminate the extension green interval before gap change if the headways are too small for a gap change but too large for efficient traffic operation. The efficiency is measured by the difference of measured space times from the headway setting. The difference is called a waste increment.
A waste timer operates throughout the running part of the phase, but its operation is ignored until the start of the extension green interval (i.e. until the end of the minimum green period or until a demand for another phase is received, whichever comes later). At the start of the extension green interval, and at each detector actuation after that, the headway timer is loaded with the headway setting. When the detector actuation ceases (i.e. at the end of the occupancy time), the headway timer starts decrementing. If the headway timer reaches zero before another actuation occurs, the timer is said to have timed out. The headway timer may time out many times during a phase.
The value of the waste timer at the start of the extension green interval equals the waste setting. Whenever the headway timer is timed out, the waste timer starts decrementing until a new detector actuation occurs. The amount of decrement equals the waste increment. When the waste timer reaches zero, the waste timer is said to have timed out. When this occurs before a gap change, the phase will be terminated. This is referred to as a waste change.
In addition to a gap change or waste change, the phase can be terminated by a minimum change, or a maximum change.
The minimum change occurs when the phase ends at the end of the minimum green period when a demand for another phase has been received and the gap timer has timed out before the end of the minimum green period.
The maximum change occurs when a gap change or waste change has not occurred during the extension green interval and the total green extension time equals the maximum extension green setting.
In summary, subject to demand for another phase, the green period can be terminated by one of four methods:
- a minimum change
- a gap change
- a waste change
- a maximum change.