6.5.3 Phasing Design
Factors to consider
Phasing design is the combination of all necessary traffic movements into a plan that describes the way movements should interrelate. The choice of phasing system depends on:
- Layout – the number of lanes and the length of left and right turn lanes available for each movement on the approach and departure of each intersecting road.
- Alignment – the horizontal and vertical alignment in regard to the angle at which roads intersect and sight distance available to allow safe filtering of right turn movements.
- Traffic flows – the amount of traffic including proportion of heavy vehicles in each through or turning movement.
- Signal coordination – progression considerations for an intersection within a coordinated system.
- Pedestrians – which pedestrian movements need to be controlled and how they will be catered for in the phasing system.
- Special vehicles – whether or not buses, trams, bicycles, emergency vehicles or (rarely) trains need to be separately controlled, and how they will be catered for in the phasing system.
- Speed environment – the operating speed of vehicles is a factor that affects both the likelihood and severity of crashes.
Phasing designs need to take into consideration safe system principles and minimise the risk of crashes which typically result in serious injuries or death. Based on data from New Zealand and Victoria, Austroads (2017a) found that the most common fatal and serious injury (FSI) crashes at urban signalised interactions are:
- Opposing-turning (right turn against)
- Adjacent (right angle)
Phasing design should also consider safety implications of intersection signal operation during low demand periods when normal phase sequencing may not occur due to phase skipping. It is desirable to have consistency in phasing arrangements both generally and at similar intersections. This is particularly important where intersections with similar characteristics and appearance are closely spaced.
Basic elements of signal phasing
The phasing arrangement at a signalised intersection is defined by the type of phasing adopted for right turning traffic. Figure 6.8 presents definitions of basic elements of phasing alternatives that are applicable to cross or T intersections and can be adapted to intersections with more than four approaches. For clarity, movements stopped by red displays are not shown in the full phase sequence diagrams. Turning movements that give way to opposing vehicle or pedestrian movements are shown as broken lines.
Crashes involving right turning vehicles and opposing through traffic pose a significant road safety risk at signalised intersections, being the most prevalent crash type resulting in fatal and serious injuries. Accordingly, full signal control of right turn movements is preferred. Filter right turns should only be used following an assessment which demonstrates that the road safety risk is low.
A risk assessment or Safe System assessment can help determine the road safety risk. This assessment should consider exposure (i.e. traffic volume), likelihood (i.e. speed, the geometry of the turn and the available sight distance) and the likely severity of a collision (i.e. speed and impact angle). Regardless of risk, the right turn must be fully controlled (no filter) where any of the following apply:
- sight distance is restricted by opposing right turning traffic
- the right turn is across more than two lanes of opposing traffic with an 85th percentile speed greater than 50 km/h
- the right turn is across more than three lanes of opposing traffic.
Where existing sites with filter right turn movements exist outside of the above requirements, a prioritisation program should be developed to reduce road safety risks.
Crashes involving pedestrians are also a significant source of FSI crashes at urban signalised intersections (Austroads 2017a). As far as is reasonably practicable, pedestrian movements should be protected by the signal phasing. This applies particularly to conflicts between right turning vehicles and pedestrians. Options for pedestrian movements include:
- A special phase for pedestrians during which all other phases are stopped and pedestrians can walk in any direction through the intersection (‘Barnes dance’ or ‘scramble crossing’).
- Full control of vehicle movements to remove conflict with pedestrians – this is the preferred control for right turn movements.
- Partial control of vehicles movements (through a delay to the start of their movement) or allow vehicles to filter through pedestrians – subject to assessment of the risks to pedestrians through road safety audit or Safe System assessment.
- Staged crossings for pedestrians where:
- it is not practicable for pedestrians to cross in one continuous movement
- pedestrian crossing time would govern the phase time and overall cycle time and this would be undesirable
- staging would provide greater flexibility in the phasing arrangements for the intersection
- there is adequate storage in the median
Audio-tactile traffic signals should be provided. Staged crossings should be used with care as they usually increase crossing delays and may therefore result in increased risk taking by pedestrians.
If the phasing arrangement includes phases for special vehicle types (such as public transport priority phases) they must not run in conflict with pedestrian phases.
Left turn and pedestrian movements are also not shown in Figure 6.8 for reason of clarity. Left turn movements do not control phase selection.
The basic phasing elements in Figure 6.8 show movements on one road only, say an East‑West road. The leading, lagging and repeat right turn phasing options are shown in terms of the right turn movement from the west approach, but they are equally applicable to a right turn movement from the east approach. Similarly, the split-approach and the lead-lag phasing options can be in reverse order (i.e. east approach first). These phasing elements can be used to build a total phasing arrangement (i.e. a complete signal cycle) for the intersection.
- Both filter right turns may be allowed (and either/or selection of the illustrated phases) subject to a satisfactory safety assessment.
- Filter right turns from the approach opposite NRT may be allowed subject to a satisfactory safety assessment.
- The leading turn must be fully controlled, and the lagging turn may be allowed to filter subject to a satisfactory safety assessment.
NRT: No Right Turn (right turn movement must be banned where opposing through movement overlaps).
For clarity, left turn and pedestrian movements are not shown.
Crash risk factors
Safety assessment and consideration of safe system principles should be undertaken when designing the geometric layout and phasing for signalised intersections. Signalised intersections that have been designed to traditional standards are not considered to be fully Safe System compliant because collisions that occur as a consequence of road user error can result in serious injury or death. The risk of serious crashes, contributing factors and alternative designs for signalised intersections have been the subject of recent studies, including Austroads (2015 and 2017a), Turner et al (2012) and Durdin et al (2016). Findings from these studies include:
- Right turn against, adjacent direction, pedestrian and same direction (rear end and side swipe) crash types are the most prevalent FSI crashes at signalised intersections
- Crash rates increase as the number of legs increases
- Crash rates increase with an increase in speed limit
- The risk of serious right turn against crashes is generally higher if:
- there is more than one opposing through lane
- right turn vehicles are allowed to filter
- signal visibility is poor
- there are no mast arms
- the intersection is large
- the degree of saturation is high
- The risk of serious adjacent direction crashes is generally higher if:
- the intersection is large and has wide approaches
- there is a lack of right turn control
- the signals are coordinated with an upstream intersection
- there are no mast arms
- there are fewer than five signal displays
- The risk of serious pedestrian crashes is generally higher if:
- right turn movements are not fully controlled or banned
- the intersection is large
- the angle of skew of the intersection is more than five degrees from perpendicular
- the signals are coordinated with an upstream intersection (applies to right turning vehicle / pedestrian crashes)
- The risk of serious rear-end rashes is generally higher if:
- there are more approach lanes
- split phasing is used
- there are shared right turn / through lanes
- the speed limit is high
For all phasing options shown in Figure 6.8 (except the split-approach phasing), providing an exclusive right turn lane is recommended in order to:
- reduce the exposure to the rear end conflict between through and right turn vehicles
- avoid lane blockage by vehicles waiting for gaps or stopped by a red display
- isolate detection of right turn vehicles to prevent through vehicles unnecessarily calling the turn phase for leading right turn phasing.
Where a right turn is banned for part of the time using a switchable electronic sign, the switching of the sign should be coordinated with the signal displays in order to obtain a safe transition. If the right turn movement is arrow controlled, the sign should preferably switch on at the same time as the arrows change from a yellow display to a red display or red arrow drop-out.
The following are examples of basic phasing systems. However, phasing arrangements can be complex in many situations, including:
- large multi-legged intersections where some vehicular movements may have to be staged within the intersection
- interchange terminals where other roads are in close proximity to the ramps (e.g. side street or frontage roads)
- where special public transport (bus and/or tram) phases or emergency services phases have to be incorporated into the system.
While Figures 6.9, 6.10 and 6.11 show filter right turns, full control of right turn movements is preferred and should be the default phasing for right turns. Filter right turns may be permitted (with or without a right turn phase) following an assessment which indicates that the risk of FSI crashes is low having regard to the crash risk factors list above.
Appendix E presents an example of signal design comparing the impact of different traffic signal phasings, as well as other design changes.
Two phase system
The simplest signal phasing at an intersection involves two through phases with filter turns and parallel pedestrian movements as illustrated in Figure 6.9. For clarity, movements stopped by red displays are not shown in the full phase sequence diagrams. Turning movements that give way to opposing vehicle or pedestrian movements (filter turns) are shown in broken lines. By allocating right of way to each road alternately, the two-phase system eliminates all crossing conflicts between through traffic movements but retains 16 other conflict points (Commentary 6).
Three phase system – leading right turns
Figure 6.10 shows a three phase system for a leading right turn at a cross intersection. Leading right turns are generally favoured because opposing traffic is stopped when the right turn phase starts, resulting in safer operation. Leading right turns are often provided on all intersection approaches and may run concurrently from opposing directions. The use of a leading right turn phase results in a three phase system.
Three phase system – lagging right turn
With lagging right turn phasing, a ‘right turn trap’ situation can occur and lead to crashes. This is because the filtering right turn vehicles would face a yellow circle display while the oncoming through traffic (from the west approach in Figure 6.11) faces a green circle display during the phase transition (from Phase A to Phase B). This situation also applies to T-intersections with filter U‑turns (where permitted under local road rules).
In this case, a driver who wants to turn right by filtering at the end of the first phase from the direction opposing the lagging right turn (right turns from the east approach filtering at the end of Phase A in Figure 6.11) will see the signal display changing to yellow. The driver may think that the signals change to yellow for the opposing traffic from the west approach in Figure 6.11 at the same time, and therefore proceed and run into an opposing through vehicle for which the signal display would still be green (‘right turn trap’).
If the right turn movement from the direction opposing the lagging right turn cannot be banned, this conflict situation must be avoided by:
- using a leading right turn sequence (Figure 6.10)
- forcing the overlapping through movement (from the west approach in Figure 6.11 to stop and then start up again (though this is not an efficient method)
- using another phasing such as split-approach phasing, diamond overlap phasing, or lead‑lag right turn phasing as shown in Figure 6.8.
Summary of types of right turn phases
|Type of phasing||Description||Signal display||Comments|
|Through phasing with filter right turns||The through and left turn movements and filter right turns from opposing approaches operate in the same phase (Figure 6.8 and Figure 6.9).||Three-aspect circular (red, yellow, green) signal faces.|
In general, filter right turn movements should be avoided due to the inherent risk of collisions with opposing through traffic (including motorcyclists and bicyclists) and / or pedestrians. A filter right turn (including partial filtering in conjunction with a green right turn arrow) should only be considered after:
If a suitable phasing alternative that can cater for the right turn movements in an efficient and safe manner cannot be found, consideration should be given to banning right turns.
|Leading right turn||The right turn phase precedes the phase in which the opposing through movement runs (refer to Figure 6.8 and Figure 6.10). In Figure 6.10 the leading right turn movement from the west approach runs in Phase A, and the opposing through movement from the east approach runs in Phase B.|
Three-aspect right turn arrows (red, yellow, green) in a six-aspect signal face,
two-aspect right turn arrows (yellow, green) in a five‑aspect signal face.
|Split approach||Allocates separate phases to opposing approaches at four-way intersections (Figure 6.8). The through and turning movements from each approach operate simultaneously, and right turn movements are unopposed under this phasing.||Split-approach phasing is controlled by four aspect signal faces, i.e. three circular aspects (red, yellow, green) and a green arrow aspect.|
Split-approach phasing may also be appropriate where:
Split phasing has significant safety benefits compared to signal phasing options that permit filtered right turn movements at any time during the signal cycle. However, split phasing may reduce the efficiency of site operation and can increase rear-end crashes.
|Diamond overlap||Allows right turns from opposing directions to operate either simultaneously or independently with the through movement on the same approach, depending on demand for the right turns and conflicting through traffic on the road controlled by the diamond overlap phasing in each signal cycle (Figure 6.8).|
Diamond overlap phasing is controlled by:
|Lead-lag right turn||Combines the leading and lagging right turn arrangements, that is, a right turn phase precedes the phase in which both through movements run followed by a right turn phase for the right turn movement from the opposing approach (see Figure 6.8).|
In this phasing the leading right turn must be fully controlled using three-aspect right turn arrows (red, yellow, green) in a six aspect signal face.
The lagging right turn should also be fully controlled using three-aspect right turn arrows (red, yellow, green) in a six aspect signal face, or may in some circumstances be partially controlled using either three-aspect right turn arrows with red arrow drop out or two-aspect right turn arrows (yellow, green) in a five-aspect signal face.
|Repeat right turn||Introduces the arrow-controlled right turn twice in the same cycle (see Figure 6.8) Effectively, this provides a combined leading and lagging right turn arrangement for a selected right turn movement, unlike the lead-lag right turn phasing that applies to the right turns from opposing directions.|
The right turn associated with the repeat phasing should generally be fully controlled using three-aspect right turn arrows (red, yellow, green) in a six-aspect signal face.
In some circumstances, partial control using either three-aspect right turn arrows (red, yellow, green) with red arrow drop out or two-aspect right turn arrows (yellow, green) in a five-aspect signal face may be acceptable.
The right turn from the opposite approach to the repeat phase must be either: