Synergistic Traffic Intersection

Synergistic Traffic Intersection

Abstract
At traffic intersections the right-turning vehicles cause conflict with the through traffic triggering a discontinuity and low efficiency in traffic flow. This paper presents the Synergistic Traffic Intersection, a proposed technique to improve traffic flow, discussing the design concepts of the proposed intersection and conducting experiments to test the efficiency differences when associated with different traffic volumes. Micro-simulation experiments are conducted using VISSIM, which have demonstrated a major improvement in traffic efficiency, concluding that although more research is required to be conducted on the risks associated with human error, the Synergistic Traffic Intersection will serve as a contender to the Conventional Intersection. 

Introduction
Traffic congestion is mainly caused by the conflict of right-turning vehicles crossing the path of through traffic. Conventional traffic management techniques suggest prohibiting right-turns, introducing a specialised right-turning phase, and the use of one-ways roads, but this does not eliminate the spatial and temporal conflict points effectively, suggesting that Conventional Intersections (CI) is limited in its ability to filter traffic efficiently.

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The Synergistic Traffic Intersection (STI) is designed to reduce the intersection delay, by relocating the right-turning manoeuvre. A right-turn lane is placed to the right of oncoming traffic. The right-turning vehicles enter this lane through of a midblock intersection. Which is a signalised intersection that allows vehicles to load the right-turning lane, crossing the oncoming traffic lanes during a signal phase that services the cross street traffic, consequently eliminating the conflict. Resulting in an intersection that is able to service the through traffic whilst also providing a protected right-turn movement. Because of this, all directions of traffic can be serviced within two phases. A two phased system allows for better coordination of traffic lights for an even smoother flow. Using this concept, we can significantly reduce congestion compared to its counterpart CI model.

A viable solution to the traffic congestion problem must satisfy the following criteria. Firstly, the solution must be economically achievable, it must not involve large construction or conversion costs. Secondly, the solution must be quick to construct, limiting the impact caused to road users. Lastly, must demonstrate a significant improvement in efficiency, where the benefits are clear for road users to support and be willing to adopt. The STI design, involves only minor changes to the civil architecture, changes to traffic signals and road markings. This design will be cost effective and relatively quick to construct. Unlike other solutions that might be limited by the available space, the STI requires equal or less space when compared to the CI design, and possess little changes to current driving behaviours.

This article presents the STI as a solution to the urban road congestion problem by firstly considering the characteristic of the CI and comparing it to STI design. The design of the STI including the traffic space design, the traffic organization design and the traffic control design. Experiments comparing the performance of the CI and corresponding STI design is validated via VISSIM. Finally, after considering the characteristics of the traffic intersection design and road design requirements. Conclude on the suitability for STI as a viable solution to the urban traffic congestion problem.

Preliminary
Geometric Model of the STI.
A mid-block intersection is placed in the road at a practical distance from the intersection shown in Fig.1. A mid-block intersection will be used as a shared space to relocate right-turning vehicles to eliminate interference with through traffic.
Right-turning traffic gathers in the sheltered right-turn lane before the mid-block intersection (Lane 4), providing sufficient space to cross the opposing oncoming traffic towards the farthest right-side lane (Lane 6), prior to the main intersection in preparation of the right-turn. The right-turning vehicles will then join the north-bound traffic as they pass through the main intersection into the receiving lane of the north-bound traffic.

Left-turning vehicles proceed past the mid-block intersection using lane 1, remaining in lane 1 and joining the south-bound traffic as they pass through the main intersection into a receiving lane of the south-bound traffic.

The through traffic will proceed past the mid-block intersection using lane 2 and 3, remaining in the same lane as they proceed straight through the main intersection into a receiving lane of the west-bound traffic. 

DesignConcept

A CI uses a 4-phased system, which allows the vehicles travelling to and from 4 different directions to proceed through the intersection. This causes vehicles to wait for traffic to cycle through 3 other directions before allowing them to continue through the intersection, resulting in a 3:1 wait time ratio. The STI displaces the right-turning vehicles to the farthest right-side lane well before approaching the intersection, enabling the right-turning vehicle and the oncoming through vehicles to move simultaneously without conflict. Vehicles travelling to and from 4 directions can proceed through the intersection within 2-phases, resulting in a 2/3 reduction in the waiting time and a wait time ratio of 1:1.

A “Phantom Bottleneck” is caused by a slow-moving vehicle that occupies a lane on a road, often causing a queue behind the vehicle (Gazis et al. 1992). The STI model attempts to decrease this effect by introducing more departing lanes than receiving lanes. For example. In a 6 x 6 STI model (6-lane road intersects another 6-lane road), there is 2 receiving and 4 departing lanes, the 2 receiving lanes expand from lane 2 to 3 and expands to lane 4 as vehicles proceed to another intersection. This acts like an anti-funnel, which allows vehicles entering the intersection additional lanes to increase speed and to bypass slow moving vehicles.

Under CI conditions the right-turning vehicles gather in the middle of the intersection waiting for either a specialised right-turning phase or breaks in the opposing traffic flow to safely turn right. This gathering of vehicles can severely interrupt straight vehicles from proceeding through the intersection and can cause safety hazards with oncoming traffic. The STI gathers right-turning vehicles at a practical distance before the intersection in a sheltered right-turn lane with traffic signals that indicate when it’ll be safe to cross the oncoming traffic through the mid-block intersection into the right-turning lane. This ensures that the interference from the right-turning traffic and through traffic is kept to a minimal.

As right-turning vehicles turn from the right-side lane, larger vehicles with more axles such as busses and trucks would have difficulty in their ability to successfully complete the turn. The STI has introduced a wedged shaped clear zone ensuring that any vehicle can successfully complete their right or left turn, without obstructing other road furniture and users. As a consequence, vehicles waiting before the main intersection will stop on a diagonal stop line in a staggered like fashion.

The STI ensures the improvement of traffic operation whilst retaining the safe and frequent crossing of pedestrians. Pedestrians are allowed to cross at times when there are no vehicles turning through the crosswalk. After crossing one leg of the intersection, pedestrians will receive other signal phase that allows them to cross another leg of the intersection to end up in a diagonal position from the point the pedestrian started.

Principles of design

It is imperative to consider the spartial and temporal aspects when considering the effectivness in the STIs ability as a safe and reliable traffic intersection. These design principles to be considered include:

1)      Design principles of space: a rational and effective distribution of traffic facilities that ensure safety.

2)      Design principles of traffic organization: The ability to spead the conflict point or reduce conflict points.

3)      Design principles of traffic control: Arterial coordination control.

Design of the Domestic STI

Traffic space design.

1)      Design of mid-block intersection.
The mid-block intersection is placed at a distance of L Meters from the main intersection. As the distance of L increases the desired effects will worsen. The distance should match the arterial coordination control strategy. The value of L is determined by the distance between intersections, the desired queue length and the average speed.
 

2)      Design of main intersection
The design of the main intersection should include the lane width and the number of lanes. The lane widths should be determined by the road and traffic conditions and the country or location of the intersection. The number of lanes can be either determined by the desired capacity or limited to the space that’s available. 

3)      Recommendation of parameters values
The distance of L between the main intersection and mid-block intersection is 75m and the length of the mid-block intersection to be 50m. The sheltered right turn lane before the mid-block is 60m with a taper of 20m, and the lane widths are 3.5m (Austroads, 2017).
Please note that these are recommended parameters only, these figures can be lowered or increased according to local traffic laws and guidelines, and dependent on the constrains that might exist.

4)      Swept path analysis.
A swept path analysis is used to ensure that STI provides sufficient space and room for turning vehicles to successfully compete the turn without damaging roadside furniture or other road users. Please see appendix for swept path illustrations.

Traffic Organization Design.

1)      Traffic organization of right-turning vehicles

1. When vehicles make a right-turn, vehicles gather under the instructions of road markings and approach the mid-block intersection in lane 3 and enter the sheltered right-turn lane in lane 4. Please refer to Fig.2.
2. At the mid-block intersection, vehicles will wait if a red traffic light is indicating that it isn’t safe to cross. When a green traffic signal is displayed the vehicles cross the opposing traffic lane and proceed right to the right-turning lane in lane 6.
3. Shortly after arriving at the right-turning lane, the vehicles will receive a green signal indicating that it is safe to turn right whilst the cross street receives a red-light indicating stop.

    

2)      Traffic organization of left-turn vehicles

1. Left turning vehicles would arrive at the main intersection under the guidance of road markings in lane 1, waiting for traffic signals to indicate when it will be safe to proceed through the main intersection and turn left.
2. After turning left into the receiving lane, vehicles will stop before the mid-block intersection, waiting for the green traffic signal to indicate that it is safe to proceed through.

3)      Traffic organization of straight vehicles

1. Vehicles that are proceeding straight will arrive at the main intersection under the guidance of road markings, waiting at the red traffic signal until the signal turns green, indicating that it is safe to proceed through the main intersection towards the mid-block intersection on the opposing side.
2. The signals at the main intersections and operate under a single switch, so straight vehicles can cross the intersection continuously.

Traffic Control Design.
Includes the distribution and coordination of traffic signals, designed signal phases and design of the coordination control of traffic signals.
 

1)     
Deployment of Signal Lights.
As in Fig.1, traffic signals are positioned at the mid-block and main intersection.

2)      Design of Signal Phases.

A 2-phased signal is used to control the intersection. When assuming the intersection is for two streets, Street α and Street β. The first phase serves the straight vehicles on Street α, meanwhile the entrance into Street β can be divided into three sub-phases. Servicing the right-turning vehicles, pedestrians and left-turning vehicles respectively. It is imperative that a sufficient amount of time be allocated so all three movements can successfully be completed. The design of signal sub-phases are illustrated in the Fig. 3. Taking sub-phase (c) & (d) for example, the right-turning vehicles on Street β, arrive at the sheltered right turn lane during sub-phase (a) & (b) waiting for the sub-phase (c) to indicate when it will be safe to cross the opposing traffic and load the right-turning lane at the main intersection, the receiving lanes are stopped by traffic signals at the mid-block intersection. In sub-phase (d) the right turning vehicles will be given a green signal shortly after arriving at the main intersection, allowing vehicles to turn right into street α.

3)      Design of Signal Phases for pedestrians.

In subphase (b) & (e) all turning vehicles are stopped to allow pedestrians to cross the road without conflict. It is important to ensure that there is sufficient time allocated in this phase to allow pedestrians to successful cross the road. For example, in Australia the average pedestrian speed is 1.2 m/s but this may vary for older or disabled pedestrians (Austroads, 2017).

If high levels of pedestrian traffic is expected, a dedicated pedestrian interval may be added, to ensure greater pedestrian safety.

4)      Design of coordination control of traffic signals
Effective coordination control ensures that vehicles cross continuously, the critical components of traffic signals is the coordination of the main and mid-block intersections. The design coordination of traffic signals should list the cycle, split and offset. The coordination control of traffic signal is separated into two groups: coordinated control based on delay and coordinated control created on a “green-band”. Green-band occurs when a sequence of traffic lights is synchronised to allow a continuous flow of traffic over numerous intersections (Webster, 1963). The STI is a two phased intersections, and therefore will be much easier to establish a coordinated group of intersections.

Case Study

Test Intersection

The micro-simulator VISSIM was used to provide an unbiased evaluator for the modelling of the CI under a four phased signal control, with specialised right turn lanes and channelised left turn lanes; And the corresponding STI design. Fig.4 shows the visual output of VISSIM micro-simulator when evaluating the CI and STI.

Simulation Scenarios

In order to observe the proposed STI under different traffic volume conditions. This study designs several scenarios, the specification of which are listed below.

1)      Assuming that each entrance lane has the same traffic demand and volume, the CI and STI traffic volumes were set to 1000 veh/h, 2000 veh/h, 3000 veh/h, 4000 veh/h, 5000 veh/h, 6000 veh/h and 7000 veh/h. Representing different traffic conditions from free to oversaturated. 

2)      Each traffic volume will be simulated for 1h, totalling 7h for each intersection design. The turning ratios of right-straight-left set to 0.25-0.6-0.15 respectively.

3)      The total cycle time will be set to 120s.

4)      To overcome the stochastic nature of the simulation results, an average of 20 simulation was used.

Results

Performance indies in this study are the average delay (Delay), and the average queue length of vehicles (Queue).

Traffic efficiency evaluation
 

The simulation results evaluating the seven traffic conditions are shown in Table 1 & 2 and in Fig.5.
Throughout the simulation process the STI shows a stable increase in Delay and Queue when compared to the CI. The CI shows a major increase in the Delay between 4000 veh/h and 5000 veh/h, whereas the STI shows a relatively stable trend.

The queue length of the CI also represents a greater increase in length after 4000 veh/h, whereas the STI show a smaller increase in Queue.

Conclusion
This article presents the Synergistic Traffic Intersection as a solution to the urban road congestion problem. And provides the STIs geometric model and design principles, along with the traffic space, traffic organisation and traffic control strategies. Conducting experiments taking the CI and corresponding STI as research objects, evaluating the performance of both CI and STI under different traffic conditions. Using VISSIM, the simulation results under seven traffic conditions have demonstrated the potential of the proposed STIs ability to improve traffic efficiency. Requiring nominal space and construction whilst ensuring crosswalk safety, the STIs ability to achieve improvements in traffic flow, proposes an alternative to all the conventional designs. Although more research will be needed to outline the risk associated with human error, this article presents the STI as a strong contender and alternative to the Conventional Intersection.
 

Acknowledgements

We thank Jo Garretty and her team at SALT3, an Australian based traffic engineering and waste engineering consultancies company, for providing a third-party evaluation of the Synergistic Traffic Intersection, for which their comments have suggested recommendation for improvements to the design, which has greatly improved the manuscript.

Reference List