Appendix G 4.3 Maximum Extension Green Setting
The maximum extension green setting is used to control the maximum green time available to each phase or signal group when conflicting demands exist and when operating in the isolated mode. This setting determines the duration of the extension green interval (Figure G 1).
Maximum green time is the sum of minimum green time (Appendix A) and the maximum extension green setting. In some controllers, a maximum green setting equivalent to the maximum green time is used.
As seen in Table G 1, typical maximum extension green settings used in practice are 10 to 85 seconds for through movement phases and 5 to 25 seconds for arrow-controlled right turn phases.
The maximum green times should be determined with the objectives of ensuring equitable distribution of green time amongst competing signal groups (movements) and achieving optimum traffic performance (e.g. minimum delay, stops or queue length), considering different traffic demand periods. This is a key parameter for optimising the performance of actuated traffic signals.
When determining maximum green times, a balance needs to be achieved between:
- erring on the long side considering that the gap setting should reduce the green time if necessary
- large maximum green time values for individual phases (or signal groups) which can add up to unduly long cycle times resulting in intolerable delay (Appendix G.2.2).
The optimum values of maximum green times produced by an appropriate software package could be used as a guide to determining the maximum extension green settings with the above criteria in mind (maximum extension green setting = maximum green time – minimum green time).
Equation A1 provides a simple manual method for determining the maximum extension green setting (Akçelik 1995b, Appendix B):
|Gemax = [y Rmax / (xp – y)] – Gmin||A1|
|Gemax||=||maximum extension green setting (s)|
|Gmin||=||minimum green time (= basic minimum green setting with stop-line detectors) (s)|
|y||=||flow ratio (demand flow rate/saturation flow rate) for the critical lane|
|Rmax||=||maximum red time that is acceptable to drivers (e.g. 60 seconds)|
|xp||=||practical degree of saturation (maximum value acceptable at high demand conditions), e.g. xp = 0.95|