Table of Contents

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)
  • Pedestrians
  • 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.

Figure 6.8: Definitions of basic elements of signal phasing

  1. Both filter right turns may be allowed (and either/or selection of the illustrated phases) subject to a satisfactory safety assessment.
  2. Filter right turns from the approach opposite NRT may be allowed subject to a satisfactory safety assessment.
  3. 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.

Phasing systems

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).

Figure 6.9: Two phase system, filter right turns and parallel pedestrian movements

Description: GTM6_53

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.

Figure 6.10: Three phase system, leading right turn phasing on east-west road

Description: GTM6_54A

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.

Figure 6.11: Three phase system, lagging right turn phasing on east-west road

Description: GTM6_55

Summary of types of right turn phases

Table 6.7 provides a summary of the types of right turn phase and their characteristics. Details of the signal displays are provided in Part 10 of the Guide to Traffic Management (Austroads 2019d).

Table 6.7: Summary of types of right turn phases and their characteristics

Type of phasingDescriptionSignal displayComments
Through phasing with filter right turnsThe 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:

  • a thorough assessment of the safety risks, and
  • assessment of the intersection performance

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 turnThe 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,

or

two-aspect right turn arrows (yellow, green) in a five‑aspect signal face.

  • This is a suitable option where an arrow-controlled right turn has a filter right turn from the opposing direction), which cannot be banned and is able to filter safely and efficiently (i.e. the right turn movement from the east approach).
  • Where it is safe to do so, the arrow-controlled right turn can be allowed to filter through the opposing through movement during the following phase (the right turn movement from the west approach in Phase B in Figure 6.10). If filtering causes safety problems, this right turn must be stopped, using a red arrow display, when the opposing through movement is operating (full control).
  • Where the leading right turn movement is allowed to filter during the following phase in the case of a six‑aspect signal face, the arrow-controlled turn must be terminated and held on a red arrow display at the beginning of the green circular display (at the start of Phase B in Figure 6.10). The red arrow is then extinguished for the remainder of the green circular display (red arrow drop out).
  • This phasing system becomes inefficient for shared lanes when a through vehicle calls the right turn phase and there are no right turn vehicles during that phase.
  • The provision of an exclusive right turn lane (turn‑bay or full‑length lane) is recommended.
Split approachAllocates 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:

  • side streets at an intersection are slightly offset so right turns cannot make a diamond turn, or sight distance makes opposing filter right turn movements unsafe
  • turn proportions vary significantly during the day requiring flexible shared lane arrangements
  • a particularly heavy right turn movement is opposed by a very light movement, in which case the right turn vehicles may fail to give way to opposing through vehicles.

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 overlapAllows 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:

  • three-aspect right turn arrows (red, yellow, green) in a six-aspect signal face

or

  • two-aspect right turn arrows (yellow, green) in a five-aspect signal face.
  • The diamond overlap phasing is used where opposing right turn flows are too large for efficient filter operation alone at four-way intersections, or there are safety reasons that preclude right turn filtering being allowed.
  • Filter right turns must only be considered after a thorough assessment of safety risks. If filter turns are used, they are introduced after both right turn movements are stopped and both through movements are started (red arrow drop out).
  • Filter right turns must not be used where either of the following apply:
    • 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
  • The diamond overlap phasing shown in Figure 6.8 provides leading right turns in both directions, thus avoiding phase transitions that cause lagging right turn conflict where filter turns are used.
  • To allow right turns to operate concurrently the swept paths of design vehicle from both approaches have to be checked to ensure sufficient clearance between vehicles
Lead-lag right turnCombines 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.

  • This phasing is useful for signal coordination purposes as right turn phases may be sequenced to optimise progression during through movement phases.
  • Lead-lag phasing provides less flexibility and efficiency for managing delays and queues (than diamond overlap phasing) and may not be desirable at locations where coordination is not critical.
  • The lagging right turn movement should be fully controlled using a red arrow to minimise the safety risk and for consistency of right turn control. Filtering of the lagging right turn should only be permitted following a thorough assessment of the safety risks.
  • The leading right turn movement must not be allowed to filter during the following through phase (i.e. should be fully controlled) in order to avoid the lagging right turn conflict discussed previously.
Repeat right turnIntroduces 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:

  • banned

or

  • fully controlled using three-aspect right turn arrows (red, yellow, green) in a six-aspect signal face.
  • The right turn movement subject to the repeat phasing may be allowed to filter during the through phase (i.e. be partially controlled) after a thorough assessment of the safety risks.
  • The right turn movement from the opposing direction should not be allowed to filter during the through phase (i.e. should be fully controlled) in order to avoid the lagging right turn conflict discussed previously.
  • This phasing can be used for either full-time or part-time operation. Its use will depend on the degree to which right turn traffic flow fluctuates at a site.
  • Repeat right turn phasing increases short lane capacities by using two short green intervals (applicable to right turn bays). It is also useful where there is insufficient storage in the road into which the right turning vehicles are entering due to congestion. In this case, this phasing operates as a metering device.
  • The use of a repeat right turn phase introduces an additional inter-green period that may impact the efficiency of the intersection if the right turn is a critical movement.