- Email: [email protected]
Simulating transit priority: Continuous median lane roundabouts
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
ScienceDirect ScienceDirect
Procedia Computer Science 00 (2017) 000–000 Available online at www.sciencedirect.com Procedia Computer Science 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Computer Science 109C (2017) 849–854
The 6th International Workshop on Agent-based Mobility, Traffic and Transportation Models, The 6th International Workshop on Agent-based Mobility, Traffic and Transportation Models, Methodologies and Applications (ABMTRANS) Methodologies and Applications (ABMTRANS)
Simulating transit priority: Continuous median lane roundabouts Simulating transit priority: Continuous median lane roundabouts Erlend Aakreaa *, Arvid Aakreaa Erlend Aakre *, Arvid Aakre
NTNU Traffic Engineering Research Centre, 7491 Trondheim, Norway NTNU Traffic Engineering Research Centre, 7491 Trondheim, Norway
a a
Abstract Abstract This paper describes and analyses a layout and regulation, which provides 100 % bus priority and no delays for buses at small and This paper describes and analyses a layout and regulation, which provides 100 %isbus priority and no delays forrapid busestransit at small and medium sized roundabouts. The Continuous Median Lane Roundabout (CMLR) designed specifically for bus (BRT) medium Continuous Median Roundabout (CMLR) is designed forsolutions, bus rapid access transit (BRT) or busessized with roundabouts. high level ofThe service (BHLS), whereLane buses run in median exclusive lanes.specifically In previous to the or buses with high level of service run inlanes median exclusive lanes.by Intraffic previous solutions, the roundabout circulating lanes from the(BHLS), general where purposebuses approach has been controlled signals. In the access CMLR,togive roundabout circulating lanes from control the general purpose approach has been by traffic In the CMLR,ingive way signs within the roundabout the two conflict pointslanes between busescontrolled in the median lanessignals. and other vehicles the way signs within the roundabout control the there two conflict points betweenpotential buses inforthe median lanesatand other vehicles in the circulating lanes. Microsimulations show that is a large and unused transit priority roundabouts. Significant circulating lanes. Microsimulations show there is a large and unused potential for transit priority at roundabouts. Significant reductions in delays and emissions may bethat achieved. reductions in delays and emissions may be achieved. © 2016 The Authors. Published by Elsevier B.V. 1877-0509 2017 The Authors.by Published by Elsevier B.V. © 2016 The©under Authors. Published B.V. Peer-review responsibility of Elsevier the Conference Conference Program Chairs. Chairs. Peer-review under responsibility of the Program Peer-review under responsibility of the Conference Program Chairs. Keywords: Micro-simulation; transit priority; roundabout Keywords: Micro-simulation; transit priority; roundabout
1. Introduction 1. Introduction Most major cities, at certain times, experience transport network capacity problems. Oversaturation leads to increased Most cities, atascertain times, network capacity problems. Oversaturation leads to The increased queuesmajor and delays, well as beingexperience a threat totransport economical and environmentally sustainable development. most queues and delays, as well as being a threat to economical and environmentally sustainable development. The most efficient and sustainable way to transport people in cities is usually by public transport. Reducing emissions from efficient and sustainable way to transport people in cities is usually by public transport. Reducing emissions from transport is an important issue in Norway. A harsh climate, hilly terrain and having the third lowest population density transport an important issue in Norway. A harsh climate, hilly terrain and having the third lowest population density in Europeishardly facilitates environmentally sustainable transport of people and goods. Nonetheless, the Norwegian 1 in Europe hardly facilitates environmentally sustainable transport of people and goods. Nonetheless, the Norwegian National Transport Plan for 2018-2029 has ambitious environmental targets for domestic transport : National Transport Plan for 2018-2029 has ambitious environmental targets for domestic transport1:
* Corresponding author. Tel.: +47 932 48 754 * Corresponding Tel.: +47 932 48 754 E-mail address:author. [email protected] E-mail address: [email protected] 1877-0509 © 2016 The Authors. Published by Elsevier B.V. 1877-0509 2016responsibility The Authors. of Published by Elsevier B.V. Chairs. Peer-review©under the Conference Program Peer-review under responsibility of the Conference Program Chairs.
1877-0509 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Conference Program Chairs. 10.1016/j.procs.2017.05.400
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Erlend Aakre et al. / Procedia Computer Science 109C (2017) 849–854 Erlend Aakre & Arvid Aakre / Procedia Computer Science 00 (2017) 000–000
50% reduction of emissions from transport Zero growth in number of trips made by private cars in cities; which means that all growth in transport has to be made by public transport, bicycle or by foot.
Political incentives have made Norway the largest market for electric vehicles in Europe2. The reasons are tax benefits, free parking, free charging, no tolls, access to bus lanes etc. However, all private cars are covered by the zero-growth objective. Most cities have a road tolling system. At least 50% of the income from tolls are used to facilitate public transport, cycling and walking. Trondheim was the first Norwegian city to sign a mutual binding agreement with the central government. Extensive central funding to transport projects will be awarded if the zero-growth objective is fulfilled. In 2016, a paradoxical situation has occurred: The incentives for replacing fossil fuel cars with Zero Emission Vehicles (ZEV, vehicles with no tailpipe pollutants from onboard source of power) are (too) effective; car use is increasing. In order to achieve zero growth in private car-use, it is important to develop effective transit priority schemes. A new bus system is part of Trondheim’s plan. 2. Existing solutions for transit priority in roundabouts Buses with High Level of Service (BHLS) is the European equivalent to the more famous Bus Rapid Transit (BRT) concept. Both BHLS and BRT are high-quality bus systems, designed for efficient and comfortable transit. The definitions and characteristics are similar, but because of the differences in urban planning in Europe and America, BHLS and BRT may differ in practice3. The European BHLS is defined in the following way4: The Bus with High Level of Service is a bus-based system, clearly identified, that is an element of the primary public transport network. It offers to the passenger a very good performance and comfort level, as a rail-based system, from terminus to terminus at station, into vehicle and during the trip. The “system” approach across infrastructure, vehicles and operating tools have coherent and permanent objectives in accordance with the mobility network and city context. Traditionally, signal control has been the preferred intersection type in American BRT systems. This line of thinking is clearly demonstrated by Eccles and Levinson, who state that “median busway intersections should always operate under signal control”5. The Bus Rapid Transit Planning Guide by Wright and Hook6 provides a thorough review of the Bus Rapid Transit (BRT) field. Possible intersection designs with bus priority are covered extensively. They mention two main categories of solutions; signalized intersections or roundabouts. Furthermore, they claim that normal roundabout operation may cause considerable uncertainty for the busway system. Five possibilities for accommodating BRT systems through a roundabout are presented: 1. 2. 3. 4. 5.
Mixed traffic operation Mixed traffic operation with signalized waiting areas Exclusive lane along inside of roundabout Exclusive busway through the middle of the roundabout Grade separation
Referring to option 4, with exclusive busway through the middle of the roundabout, they state that “in this design, a traffic signal controls enter to and from the roundabout”. Several examples of such solutions exist throughout the world, one of them being the roundabout at Hillevåg, Stavanger. In fact, examples of transit priority at roundabouts usually include traffic signals in some way. At a fully signalized roundabout, all movements and conflicts are controlled by traffic signals. There are signals at all entries and also for circulating traffic inside the roundabout. Each roundabout entry could be seen as a T-intersection of two one-way streets. To avoid long queues (and blocking) inside the roundabout, these roundabouts need to be rather large. The cycle time should be low, and all signals at the roundabout should be coordinated. At a partly signalized roundabout, some entries have signals and some have traditional give-way sign priority. This could be more flexible than a fully signalized roundabout.
Erlend Aakre et al. / Procedia Computer Science 109C (2017) 849–854 Erlend Aakre & Arvid Aakre / Procedia Computer Science 00 (2015) 000–000
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Some roundabouts have unbalanced flows, and we might get a situation with very long queues and delays at one or more entries. The reason is that we have a dominant and continuous flow which blocks entry from another movements. The solution might be to introduce roundabout metering; the dominant flow is stopped by a traffic signal some distance before the entry point in order to meter the entry flow. Apart from this metering, the roundabout operates as a conventional roundabout. 3. A new method for bus priority at roundabouts without traffic signals The concept of a roundabout that provides 100 % transit priority without traffic signals was recently presented by Aakre7. The Continuous Median Lane Roundabout (CMLR) seeks to achieve the absolute bus priority warranted by traffic signals, while at the same time keeping the flexibility and high capacity of a roundabout. The continuous median lane (CML) is the starting point of the CMLR. This underlines the objective of providing 100 % priority to buses. Subsequently, all the general purpose lanes (GPL) are added. The buses have exclusive access to the CMLs, and they do not yield to any other movements. Cars entering the roundabout from the GPL act as in a normal roundabout, by yielding to conflicting circulating movements. In addition, give way-lines in the circulating area are placed by the CML exits; when buses use the CML through the central island, circulating cars must yield before they cross the CML. When no buses are present, the CMLR operates as a normal roundabout. Two different versions of the CMLR are presented in Figure 1:
Figure 1 Schematic illustrations of CMLR. Simplified solution (A) and full solution (B)
The CMLR in its simplest form is presented in Figure 1 (A). Bus stops may be located in the central island or in the CML, preferably downstream from the roundabout. If more capacity is needed, the full solution in Figure 1 (B) can be used. The full solution includes extra lanes for movements with bus conflicts (through and left turn from the East and West approaches, and left turn from North and South approaches). Bus detectors may be used to enhance safety if deemed necessary. If a bus is detected before approaching the roundabout, drivers in the general purpose lanes with bus conflicts (purple) can be warned about the potential conflict. Different-colored surfaces can be used together with variable message signs or light signals to make the drivers in the circulating area aware potential conflicts with buses. 4. Microsimulations and results To analyse the effect of replacing a signalized roundabout with exclusive busway through the middle of the roundabout (option 4 in the Bus Rapid Transit Intersections section) with a CMLR, field observations and microsimulations of an
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Erlend Aakre et al. / Procedia Computer Science 109C (2017) 849–854 Erlend Aakre & Arvid Aakre / Procedia Computer Science 00 (2017) 000–000
intersection at Hillevåg (Stavanger, Norway) have been carried out. The field observations are reported in a Master thesis by Bråtveit8. 4.1. Microsimulations The current design at Hillevåg is shown in Figure 2 (A), while (B) shows the same roundabout with the CMLR-design.
Figure 2 Aerial photo of Hillevåg: Existing solution (A) and CMLR (B)
In Figure 2 (A), traffic signals restrict access to the roundabout for all movements but the bus. This is the existing regulation at Hillevåg. In Figure 2 (B), conflicts with the bus are regulated by yield signs inside the circulating area (CMLR). Manual traffic counts from video recordings are summarized in Table 1. Table 1 Demand matrix [veh/h] West South West South East North Total
29 20 32 81
12 179 484 675
East
North
Total
24 123
44 468 160
80 620 359 640 1699
124 271
672
The numbers represent the total vehicle demand for the peak hour in the general purpose lanes (including 7 % heavy vehicles). In addition, the median lanes carry two buses every 5 minutes (one in each direction). A peak flow factor of 0.82 was used, with two peak flow periods 15 minutes. In short this means that the two peak 15 minute periods have a demand that is 1 / 0.82 = 1.22 times as high as the peak hour average. See for instance Rouphail & Akcelik9 for more on peaking factors. The demand matrix was spilt into four 15-minute parts (high-low-high-low). In microsimulation, calibration usually consists of adjusting parameters in order to obtain simulated traffic outputs that match observed traffic data, as described by for instance Kim, et.al10. The alternative would be to calibrate the input parameters themselves, so that they match observed vehicle- and behavioural parameters. First, the current design at Hillevåg was modeled. The vehicle parameters (length and spacing) and behavioural parameters (reaction times, gap acceptance parameters and driving speeds) were calibrated with respect to the observed capacity and delay for the different intersection approaches. Subsequently, the signalized transit priority system of the current design was removed from the model, and replaced with the simplified CMLR design. For each case, 10 replications were carried out.
Erlend Aakre et al. / Procedia Computer Science 109C (2017) 849–854 Erlend Aakre & Arvid Aakre / Procedia Computer Science 00 (2015) 000–000
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Delays and emissions with the two different regulations have been simulated in the microscopic simulation tool Aimsun 8.111,12. The difference in driver behaviour between the two regulations is shown in Figure 3.
Figure 3 Microsimulation of roundabout at Hillevåg: Existing solution (A) and CMLR (B)
In Figure 3 (A), all general purpose lanes are given a red signal when the bus is detected upstream from the roundabout. By the time the bus arrives, no other vehicles are in the roundabout, and the bus may enter without conflicts. In Figure 3 (B), only vehicles in conflict with the bus actually have to yield to the bus. The red car inside the red circle gives way to the bus, while all vehicles not in conflict with the bus proceed as normal. 4.2. Delay and emissions with two different transit priority strategies As discussed in section 4.1, two different priority strategies for roundabouts with exclusive median lanes through the central island were tested; the existing solution with signalized control of access to the roundabout and the CMLR concept. The simulated delay results in Figure 4 are the average values from the set 10 of replications.
Figure 4 Simulated delay in general purpose lanes with actual demand (A) and 10 % demand increase (B)
Because the bus has full priority in both designs, results on bus delays would be quite trivial (two straight, horizontal lines, close to zero). The challenge is not to achieve low delay for the bus, but rather to do so without causing unnecessary delays in the general purpose lanes. The continuous, straight, lines represent the observed average delay for vehicles in the general purpose lanes from the South (orange) and North (blue). Correspondingly, the dashed lines (- - -) show the simulated delay with the current design, i.e. a roundabout with exclusive bus lanes through the middle, and traffic signals controlling access to the circulating area. The dotted (. . .) lines indicate the simulated delay in the CMLR-case. The CMLR gives substantially lower delays, both with the actual observed demand (A) and with a theoretical 10 % demand increase (B). In essence, the CMLR has higher capacity, resulting in lower delays and more reliable travel times for all movements and vehicle types, compared to the existing solution. The simulated average delay and total emission results for all approaches (including those perpendicular to the
Erlend Aakre et al. / Procedia Computer Science 109C (2017) 849–854 Erlend Aakre & Arvid Aakre / Procedia Computer Science 00 (2017) 000–000
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exclusive bus lanes) are presented in Table 2: Table 2 Total simulated delay and emissions for intersection Delay [sec] All Car + Truck Bus
CO2 [kg]
NOx [g]
Signals
CMLR
Signals
CMLR
Signals
CMLR
77 78 5
20 20 5
815 788 27
671 645 27
2944 2706 238
2455 2216 238
The CMLR clearly leads to lower delays than the existing design. The average simulated delay for all four approaches with the CMLR is 20 seconds (95 % confidence interval for the mean was 18-22 seconds). The existing solution leads to an average delay of 77 seconds (95 % confidence interval for the simulated mean was 70-85 seconds). Simulations with variation in demand show that the CMLR also leads to more reliable travel times, compared with the signalized roundabout. Bus delay was close to zero with both solutions (some delay related to speed adjustment at intersection and deceleration and acceleration at bus stops is inevitable). Compared to a conventional roundabout, where the bus uses the circulating area, the delay with both designs is in fact negative. The CMLR also gives emission reductions of 15-20 % compared with the existing solution, because of the smoother traffic flow. 5. Conclusions The proposed design (CMLR) consists of a roundabout with continuous median exclusive lanes running through the central island. Previously, access to the roundabout from the general purpose lanes have been controlled by traffic signals. In the CMLR, conflicts between buses and other vehicles are controlled by give way-signs. Microsimulations of CMLR have shown that (close to) zero delay for buses can be maintained, while delays for other vehicles can be significantly reduced compared to existing solutions. Because of the smoother traffic flow, introduction of the CMLR in the Hillevåg case would lead to CO2 emission reductions of about 20 %. References 1. 2. 3. 4. 5.
National Transport Plan, “National Transport Plan 2018-2029: English summary”, Oslo: Norwegian Transport Agencies, 2016. ACEA, "NEW PASSENGER CAR REGISTRATIONS BY ALTERNATIVE FUEL TYPE IN THE EUROPEAN UNION," ACEA European Automobile Manufacturers Association, Brussels, 2016. S. Rabuel, "Buses with a High Level of Service (BHLS), the French Bus Rapid Transit (BRT) Concept: Choosing and Implementing the Right System," Certu, Lyon, 2010. COST, “Buses with High Level of Service – Fundamental characteristics and recommendations for decision-making and research”, 2011. K. A. Eccles and H. S. Levinson, "TCRP Report 117: Design, operation and Safety of At-Grade Crossings of Exclusive Busways," Transit Cooperative Research Program, Washington, DC, 2007.
6.
L. Wright and W. Hook, "Bus Rapid Transit Planning Guide," Institute for Transportation & Development Policy, New York, 2007.
7.
A. Aakre, "Bus priority at roundabouts," Melbourne, 2016.
8.
L. A. Bråtveit, "Master thesis: Prioritering av kollektivtrafikk i signalregulerte rundkjøringer med midtstilt kollektivfelt (Transit priority in signalized roundabouts with median bus lanes)," NTNU, Trondheim, 2016.
9.
N. M. Rouphail and R. Akcelik, "Oversaturation Delay Estimates with Consideration of Peaking," Transportation Research Record, vol. 1365, pp. 71-81, 1992.
10.
S.-J. Kim, W. Kim and L. Rilett, "Calibration of Microsimulation Models Using Nonparametric Statistical Techniques," Transportation Research Record, vol. 1935, pp. 111-119, 2005. Transport Simulation Systems, "Aimsun 8.1 User’s Manual,", Barcelona, 2015. Panis, L. I., Broexc, S., & Ronghui, L., “Modelling instantaneous traffic emission and the influence of traffic speed limits”. Science of the Total Environment, 371(1-3), pp. 270-285, 2006.
11. 12.
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Procedia Computer Science 00 (2017) 000–000 Available online at www.sciencedirect.com Procedia Computer Science 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Computer Science 109C (2017) 849–854
The 6th International Workshop on Agent-based Mobility, Traffic and Transportation Models, The 6th International Workshop on Agent-based Mobility, Traffic and Transportation Models, Methodologies and Applications (ABMTRANS) Methodologies and Applications (ABMTRANS)
Simulating transit priority: Continuous median lane roundabouts Simulating transit priority: Continuous median lane roundabouts Erlend Aakreaa *, Arvid Aakreaa Erlend Aakre *, Arvid Aakre
NTNU Traffic Engineering Research Centre, 7491 Trondheim, Norway NTNU Traffic Engineering Research Centre, 7491 Trondheim, Norway
a a
Abstract Abstract This paper describes and analyses a layout and regulation, which provides 100 % bus priority and no delays for buses at small and This paper describes and analyses a layout and regulation, which provides 100 %isbus priority and no delays forrapid busestransit at small and medium sized roundabouts. The Continuous Median Lane Roundabout (CMLR) designed specifically for bus (BRT) medium Continuous Median Roundabout (CMLR) is designed forsolutions, bus rapid access transit (BRT) or busessized with roundabouts. high level ofThe service (BHLS), whereLane buses run in median exclusive lanes.specifically In previous to the or buses with high level of service run inlanes median exclusive lanes.by Intraffic previous solutions, the roundabout circulating lanes from the(BHLS), general where purposebuses approach has been controlled signals. In the access CMLR,togive roundabout circulating lanes from control the general purpose approach has been by traffic In the CMLR,ingive way signs within the roundabout the two conflict pointslanes between busescontrolled in the median lanessignals. and other vehicles the way signs within the roundabout control the there two conflict points betweenpotential buses inforthe median lanesatand other vehicles in the circulating lanes. Microsimulations show that is a large and unused transit priority roundabouts. Significant circulating lanes. Microsimulations show there is a large and unused potential for transit priority at roundabouts. Significant reductions in delays and emissions may bethat achieved. reductions in delays and emissions may be achieved. © 2016 The Authors. Published by Elsevier B.V. 1877-0509 2017 The Authors.by Published by Elsevier B.V. © 2016 The©under Authors. Published B.V. Peer-review responsibility of Elsevier the Conference Conference Program Chairs. Chairs. Peer-review under responsibility of the Program Peer-review under responsibility of the Conference Program Chairs. Keywords: Micro-simulation; transit priority; roundabout Keywords: Micro-simulation; transit priority; roundabout
1. Introduction 1. Introduction Most major cities, at certain times, experience transport network capacity problems. Oversaturation leads to increased Most cities, atascertain times, network capacity problems. Oversaturation leads to The increased queuesmajor and delays, well as beingexperience a threat totransport economical and environmentally sustainable development. most queues and delays, as well as being a threat to economical and environmentally sustainable development. The most efficient and sustainable way to transport people in cities is usually by public transport. Reducing emissions from efficient and sustainable way to transport people in cities is usually by public transport. Reducing emissions from transport is an important issue in Norway. A harsh climate, hilly terrain and having the third lowest population density transport an important issue in Norway. A harsh climate, hilly terrain and having the third lowest population density in Europeishardly facilitates environmentally sustainable transport of people and goods. Nonetheless, the Norwegian 1 in Europe hardly facilitates environmentally sustainable transport of people and goods. Nonetheless, the Norwegian National Transport Plan for 2018-2029 has ambitious environmental targets for domestic transport : National Transport Plan for 2018-2029 has ambitious environmental targets for domestic transport1:
* Corresponding author. Tel.: +47 932 48 754 * Corresponding Tel.: +47 932 48 754 E-mail address:author. [email protected] E-mail address: [email protected] 1877-0509 © 2016 The Authors. Published by Elsevier B.V. 1877-0509 2016responsibility The Authors. of Published by Elsevier B.V. Chairs. Peer-review©under the Conference Program Peer-review under responsibility of the Conference Program Chairs.
1877-0509 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Conference Program Chairs. 10.1016/j.procs.2017.05.400
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• •
Erlend Aakre et al. / Procedia Computer Science 109C (2017) 849–854 Erlend Aakre & Arvid Aakre / Procedia Computer Science 00 (2017) 000–000
50% reduction of emissions from transport Zero growth in number of trips made by private cars in cities; which means that all growth in transport has to be made by public transport, bicycle or by foot.
Political incentives have made Norway the largest market for electric vehicles in Europe2. The reasons are tax benefits, free parking, free charging, no tolls, access to bus lanes etc. However, all private cars are covered by the zero-growth objective. Most cities have a road tolling system. At least 50% of the income from tolls are used to facilitate public transport, cycling and walking. Trondheim was the first Norwegian city to sign a mutual binding agreement with the central government. Extensive central funding to transport projects will be awarded if the zero-growth objective is fulfilled. In 2016, a paradoxical situation has occurred: The incentives for replacing fossil fuel cars with Zero Emission Vehicles (ZEV, vehicles with no tailpipe pollutants from onboard source of power) are (too) effective; car use is increasing. In order to achieve zero growth in private car-use, it is important to develop effective transit priority schemes. A new bus system is part of Trondheim’s plan. 2. Existing solutions for transit priority in roundabouts Buses with High Level of Service (BHLS) is the European equivalent to the more famous Bus Rapid Transit (BRT) concept. Both BHLS and BRT are high-quality bus systems, designed for efficient and comfortable transit. The definitions and characteristics are similar, but because of the differences in urban planning in Europe and America, BHLS and BRT may differ in practice3. The European BHLS is defined in the following way4: The Bus with High Level of Service is a bus-based system, clearly identified, that is an element of the primary public transport network. It offers to the passenger a very good performance and comfort level, as a rail-based system, from terminus to terminus at station, into vehicle and during the trip. The “system” approach across infrastructure, vehicles and operating tools have coherent and permanent objectives in accordance with the mobility network and city context. Traditionally, signal control has been the preferred intersection type in American BRT systems. This line of thinking is clearly demonstrated by Eccles and Levinson, who state that “median busway intersections should always operate under signal control”5. The Bus Rapid Transit Planning Guide by Wright and Hook6 provides a thorough review of the Bus Rapid Transit (BRT) field. Possible intersection designs with bus priority are covered extensively. They mention two main categories of solutions; signalized intersections or roundabouts. Furthermore, they claim that normal roundabout operation may cause considerable uncertainty for the busway system. Five possibilities for accommodating BRT systems through a roundabout are presented: 1. 2. 3. 4. 5.
Mixed traffic operation Mixed traffic operation with signalized waiting areas Exclusive lane along inside of roundabout Exclusive busway through the middle of the roundabout Grade separation
Referring to option 4, with exclusive busway through the middle of the roundabout, they state that “in this design, a traffic signal controls enter to and from the roundabout”. Several examples of such solutions exist throughout the world, one of them being the roundabout at Hillevåg, Stavanger. In fact, examples of transit priority at roundabouts usually include traffic signals in some way. At a fully signalized roundabout, all movements and conflicts are controlled by traffic signals. There are signals at all entries and also for circulating traffic inside the roundabout. Each roundabout entry could be seen as a T-intersection of two one-way streets. To avoid long queues (and blocking) inside the roundabout, these roundabouts need to be rather large. The cycle time should be low, and all signals at the roundabout should be coordinated. At a partly signalized roundabout, some entries have signals and some have traditional give-way sign priority. This could be more flexible than a fully signalized roundabout.
Erlend Aakre et al. / Procedia Computer Science 109C (2017) 849–854 Erlend Aakre & Arvid Aakre / Procedia Computer Science 00 (2015) 000–000
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Some roundabouts have unbalanced flows, and we might get a situation with very long queues and delays at one or more entries. The reason is that we have a dominant and continuous flow which blocks entry from another movements. The solution might be to introduce roundabout metering; the dominant flow is stopped by a traffic signal some distance before the entry point in order to meter the entry flow. Apart from this metering, the roundabout operates as a conventional roundabout. 3. A new method for bus priority at roundabouts without traffic signals The concept of a roundabout that provides 100 % transit priority without traffic signals was recently presented by Aakre7. The Continuous Median Lane Roundabout (CMLR) seeks to achieve the absolute bus priority warranted by traffic signals, while at the same time keeping the flexibility and high capacity of a roundabout. The continuous median lane (CML) is the starting point of the CMLR. This underlines the objective of providing 100 % priority to buses. Subsequently, all the general purpose lanes (GPL) are added. The buses have exclusive access to the CMLs, and they do not yield to any other movements. Cars entering the roundabout from the GPL act as in a normal roundabout, by yielding to conflicting circulating movements. In addition, give way-lines in the circulating area are placed by the CML exits; when buses use the CML through the central island, circulating cars must yield before they cross the CML. When no buses are present, the CMLR operates as a normal roundabout. Two different versions of the CMLR are presented in Figure 1:
Figure 1 Schematic illustrations of CMLR. Simplified solution (A) and full solution (B)
The CMLR in its simplest form is presented in Figure 1 (A). Bus stops may be located in the central island or in the CML, preferably downstream from the roundabout. If more capacity is needed, the full solution in Figure 1 (B) can be used. The full solution includes extra lanes for movements with bus conflicts (through and left turn from the East and West approaches, and left turn from North and South approaches). Bus detectors may be used to enhance safety if deemed necessary. If a bus is detected before approaching the roundabout, drivers in the general purpose lanes with bus conflicts (purple) can be warned about the potential conflict. Different-colored surfaces can be used together with variable message signs or light signals to make the drivers in the circulating area aware potential conflicts with buses. 4. Microsimulations and results To analyse the effect of replacing a signalized roundabout with exclusive busway through the middle of the roundabout (option 4 in the Bus Rapid Transit Intersections section) with a CMLR, field observations and microsimulations of an
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Erlend Aakre et al. / Procedia Computer Science 109C (2017) 849–854 Erlend Aakre & Arvid Aakre / Procedia Computer Science 00 (2017) 000–000
intersection at Hillevåg (Stavanger, Norway) have been carried out. The field observations are reported in a Master thesis by Bråtveit8. 4.1. Microsimulations The current design at Hillevåg is shown in Figure 2 (A), while (B) shows the same roundabout with the CMLR-design.
Figure 2 Aerial photo of Hillevåg: Existing solution (A) and CMLR (B)
In Figure 2 (A), traffic signals restrict access to the roundabout for all movements but the bus. This is the existing regulation at Hillevåg. In Figure 2 (B), conflicts with the bus are regulated by yield signs inside the circulating area (CMLR). Manual traffic counts from video recordings are summarized in Table 1. Table 1 Demand matrix [veh/h] West South West South East North Total
29 20 32 81
12 179 484 675
East
North
Total
24 123
44 468 160
80 620 359 640 1699
124 271
672
The numbers represent the total vehicle demand for the peak hour in the general purpose lanes (including 7 % heavy vehicles). In addition, the median lanes carry two buses every 5 minutes (one in each direction). A peak flow factor of 0.82 was used, with two peak flow periods 15 minutes. In short this means that the two peak 15 minute periods have a demand that is 1 / 0.82 = 1.22 times as high as the peak hour average. See for instance Rouphail & Akcelik9 for more on peaking factors. The demand matrix was spilt into four 15-minute parts (high-low-high-low). In microsimulation, calibration usually consists of adjusting parameters in order to obtain simulated traffic outputs that match observed traffic data, as described by for instance Kim, et.al10. The alternative would be to calibrate the input parameters themselves, so that they match observed vehicle- and behavioural parameters. First, the current design at Hillevåg was modeled. The vehicle parameters (length and spacing) and behavioural parameters (reaction times, gap acceptance parameters and driving speeds) were calibrated with respect to the observed capacity and delay for the different intersection approaches. Subsequently, the signalized transit priority system of the current design was removed from the model, and replaced with the simplified CMLR design. For each case, 10 replications were carried out.
Erlend Aakre et al. / Procedia Computer Science 109C (2017) 849–854 Erlend Aakre & Arvid Aakre / Procedia Computer Science 00 (2015) 000–000
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Delays and emissions with the two different regulations have been simulated in the microscopic simulation tool Aimsun 8.111,12. The difference in driver behaviour between the two regulations is shown in Figure 3.
Figure 3 Microsimulation of roundabout at Hillevåg: Existing solution (A) and CMLR (B)
In Figure 3 (A), all general purpose lanes are given a red signal when the bus is detected upstream from the roundabout. By the time the bus arrives, no other vehicles are in the roundabout, and the bus may enter without conflicts. In Figure 3 (B), only vehicles in conflict with the bus actually have to yield to the bus. The red car inside the red circle gives way to the bus, while all vehicles not in conflict with the bus proceed as normal. 4.2. Delay and emissions with two different transit priority strategies As discussed in section 4.1, two different priority strategies for roundabouts with exclusive median lanes through the central island were tested; the existing solution with signalized control of access to the roundabout and the CMLR concept. The simulated delay results in Figure 4 are the average values from the set 10 of replications.
Figure 4 Simulated delay in general purpose lanes with actual demand (A) and 10 % demand increase (B)
Because the bus has full priority in both designs, results on bus delays would be quite trivial (two straight, horizontal lines, close to zero). The challenge is not to achieve low delay for the bus, but rather to do so without causing unnecessary delays in the general purpose lanes. The continuous, straight, lines represent the observed average delay for vehicles in the general purpose lanes from the South (orange) and North (blue). Correspondingly, the dashed lines (- - -) show the simulated delay with the current design, i.e. a roundabout with exclusive bus lanes through the middle, and traffic signals controlling access to the circulating area. The dotted (. . .) lines indicate the simulated delay in the CMLR-case. The CMLR gives substantially lower delays, both with the actual observed demand (A) and with a theoretical 10 % demand increase (B). In essence, the CMLR has higher capacity, resulting in lower delays and more reliable travel times for all movements and vehicle types, compared to the existing solution. The simulated average delay and total emission results for all approaches (including those perpendicular to the
Erlend Aakre et al. / Procedia Computer Science 109C (2017) 849–854 Erlend Aakre & Arvid Aakre / Procedia Computer Science 00 (2017) 000–000
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exclusive bus lanes) are presented in Table 2: Table 2 Total simulated delay and emissions for intersection Delay [sec] All Car + Truck Bus
CO2 [kg]
NOx [g]
Signals
CMLR
Signals
CMLR
Signals
CMLR
77 78 5
20 20 5
815 788 27
671 645 27
2944 2706 238
2455 2216 238
The CMLR clearly leads to lower delays than the existing design. The average simulated delay for all four approaches with the CMLR is 20 seconds (95 % confidence interval for the mean was 18-22 seconds). The existing solution leads to an average delay of 77 seconds (95 % confidence interval for the simulated mean was 70-85 seconds). Simulations with variation in demand show that the CMLR also leads to more reliable travel times, compared with the signalized roundabout. Bus delay was close to zero with both solutions (some delay related to speed adjustment at intersection and deceleration and acceleration at bus stops is inevitable). Compared to a conventional roundabout, where the bus uses the circulating area, the delay with both designs is in fact negative. The CMLR also gives emission reductions of 15-20 % compared with the existing solution, because of the smoother traffic flow. 5. Conclusions The proposed design (CMLR) consists of a roundabout with continuous median exclusive lanes running through the central island. Previously, access to the roundabout from the general purpose lanes have been controlled by traffic signals. In the CMLR, conflicts between buses and other vehicles are controlled by give way-signs. Microsimulations of CMLR have shown that (close to) zero delay for buses can be maintained, while delays for other vehicles can be significantly reduced compared to existing solutions. Because of the smoother traffic flow, introduction of the CMLR in the Hillevåg case would lead to CO2 emission reductions of about 20 %. References 1. 2. 3. 4. 5.
National Transport Plan, “National Transport Plan 2018-2029: English summary”, Oslo: Norwegian Transport Agencies, 2016. ACEA, "NEW PASSENGER CAR REGISTRATIONS BY ALTERNATIVE FUEL TYPE IN THE EUROPEAN UNION," ACEA European Automobile Manufacturers Association, Brussels, 2016. S. Rabuel, "Buses with a High Level of Service (BHLS), the French Bus Rapid Transit (BRT) Concept: Choosing and Implementing the Right System," Certu, Lyon, 2010. COST, “Buses with High Level of Service – Fundamental characteristics and recommendations for decision-making and research”, 2011. K. A. Eccles and H. S. Levinson, "TCRP Report 117: Design, operation and Safety of At-Grade Crossings of Exclusive Busways," Transit Cooperative Research Program, Washington, DC, 2007.
6.
L. Wright and W. Hook, "Bus Rapid Transit Planning Guide," Institute for Transportation & Development Policy, New York, 2007.
7.
A. Aakre, "Bus priority at roundabouts," Melbourne, 2016.
8.
L. A. Bråtveit, "Master thesis: Prioritering av kollektivtrafikk i signalregulerte rundkjøringer med midtstilt kollektivfelt (Transit priority in signalized roundabouts with median bus lanes)," NTNU, Trondheim, 2016.
9.
N. M. Rouphail and R. Akcelik, "Oversaturation Delay Estimates with Consideration of Peaking," Transportation Research Record, vol. 1365, pp. 71-81, 1992.
10.
S.-J. Kim, W. Kim and L. Rilett, "Calibration of Microsimulation Models Using Nonparametric Statistical Techniques," Transportation Research Record, vol. 1935, pp. 111-119, 2005. Transport Simulation Systems, "Aimsun 8.1 User’s Manual,", Barcelona, 2015. Panis, L. I., Broexc, S., & Ronghui, L., “Modelling instantaneous traffic emission and the influence of traffic speed limits”. Science of the Total Environment, 371(1-3), pp. 270-285, 2006.
11. 12.