The objectives of signal phasing design are to provide safety and efficiency. As those objectives often conflict, compromises must be considered carefully. Right-turn arrows can be used for partial or full control of that movement. Left-turn arrows allow motorists to move when there is no conflict (eg, from a side street during a diamond phase operation on the main road). Arrows are also useful for partial or full pedestrian protection (eg, when a turning movement is held back for at least part of the time for parallel pedestrians crossing the road). Left-turn arrows are used to indicate to motorists when they have to give way to opposing right-turn traffic. The left-turn arrows are in the OFF state when filtering is permitted, and the left-turn green arrow is illuminated when no opposing right-turn or pedestrian movement is allowed.
There are a variety of reasons why, in some circumstances, a lag right turn at cross intersections is less safe than a lead right turn, and the default should be a lead right-turn arrangement (refer to section 4). A lag right turn may be dictated by coordination requirements or when insufficient width does not allow opposing right-turning movements to occur simultaneously.
Particular care must be taken to avoid phasing an unintentional lag right turn when the opposing rightturn movement may filter-turn. This ‘right-turn trap’ happens when a filter right turner (or a u-turner at a T intersection) faces a yellow circle display but the opposing through movement overlaps to a lag right-turn phase. The right turner (or u-turner at a T intersection) has no indication whatsoever that the opposing through movement is not being terminated and may turn during the amber believing the opposing through movement is stopped. This situation can occur when intervening phases are skipped, eg, during low-demand periods.
Figure 11 Bad practice of allowing protected turns into their own lanes simultaneously
Figure 12 Avoid a ‘right-turn trap’ for u-turners on the north approach by either banning that u-turn or ensuring that A phase is never followed by B phase
Only experienced signal designers and operators are likely to be aware of problems with turn arrow logic. Expert assistance is required in the design stage for both the physical intersection layout and phasing design. After installation, the on-street operation should be checked by a suitably experienced signal engineer. Only suitably knowledgeable and experienced engineers should be used for compiling the controller personality.
Where an opposing right-turning movement is held on a red arrow, a green left-turn arrow (when present) should indicate to approaching motorists that they have priority over the opposing right-turn movement. This is especially important when the opposing movement is allowed to filter turn some of the time, in which case the left-turn display should be in the OFF state. Similarly, where the signal hardware is available, a green left-turn arrow should be displayed to the side street traffic when the main street operates a protected right turn.
When a left turn is associated with a right-turn phase, it is good practice for the left-turn detector to also call the right-turn phase. This results in greater efficiency, as just one of the main road approaches needs to be stopped whenever there are only left-turn vehicles.
There are some standard operating procedures for turn arrow logic (Austroads Part 7, 2003), including holding the right-turn red arrow for five seconds before dropping it when changing from a protected turn to a filter turn, and bringing a red arrow up at the same time as the adjacent full red.
The following safety and efficiency issues can arise:
Figure 13 Walk and green left-turn arrow displayed simultaneously
Figure 14 A left-turn demand on the side street should call the associated main road right-turn phase
