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DEMONSTRATION OF THE INTERMITTENT BUS LANE IN LISBON
DEMONSTRATION OF THE INTERMITTENT BUS LANE IN LISBON José Viegas a, b, Baichuan Lu a,c, João Vieira a , Ricardo Roque a a: Department of Civil Engineering, and CESUR, Instituto Superior Tecnico Av. Rovisco Pais, 1049-001 Lisboa Codex, Portugal b: TIS.pt, consultores em Transportes, Inovação e Sistemas, s.a. Av. 5 Outubro 75-7º - 1050-049 Lisboa – Portugal c: Chongqing Jiaotong University, Chongqing 400074, China Emails: [email protected], [email protected], [email protected] [email protected]
Abstract: This paper introduces the demonstration of a new bus priority measure, the Intermittent Bus Lane, in Lisbon. The situations around the chosen testing area, the arrangements and installations of the related signals, signs and other system equipment all have been shown. The detections and estimations of traffic flow and bus movements, and the control process are also described. Finally some analysis on the field data about the obtained bus priority and the influences on the general traffic flow are also given. Copyright © 2006 IFAC Keywords: Public Transport, Integrated Traffic Control, Detector, Bus Priority, Estimation.
1. INTRODUCTION
2. BASIC DESCRIPTIONS OF THE SYSTEM
Intermittent Bus Lane (IBL) is a new bus priority measure. Its concept was first introduced by Viegas, (1996), then it has been further developed theoretically (Viegas and Lu, 2001, 2004). In these papers, its concept, structure, operations, the movements of buses and other vehicles on IBL, modelling, and optimal control equations etc. all have been described in details.
After having investigated many possible areas for the IBL demonstration in Lisbon, including the geometry, the real traffic flow situations and bus movements, seven favourite places for IBL application were identified. Finally, after discussion with the Municipality and Carris, the site at the Alameda da Universidade was selected as the first IBL demonstration area.
Normally since theoretical research can not include all possible practical situations, a real world application is necessary to act as a prototype. In order to do that, the Demonstration project of the IBL in Lisbon has been launched, in which the Municipality of Lisbon, the urban bus operator (Carris) and the School of Engineering of the Technical University of Lisbon (Instituto Superior Técnico) are all involved. This paper presents the main aspects of this demonstration project: the choice of the demonstration area; the arrangements of the related signals, signs, detectors and further system equipment within the chosen area; the detections and estimations of traffic flow and bus movements, and the control process; finally some analysis on the field data about the obtained bus priority and the influences on general traffic flow.
The map around this demonstration area is shown as in Fig. 1. At I1, a very big intersection is located. From point a0 to the stop-line of I1 a relatively long (800 m) stretch of one way road exists, with two straight alignments connected by an “S” curve roughly at the middle. Along the stretch, between a1 and a13, there are two lanes. From a13 to a14 there are three lanes where one lane can be used only by the right-turn moving vehicles, then from a14 to I1, there are four lanes, and one of them is the (permanent) Bus lane. Normal traffic signal lights are operating at I1, but from a0 to I1, there are no other traffic lights. At b1 and b2 two bus stops are located. In total four bus lines pass through these two bus stops, with a frequency of about 18,2 bus/h.
© 11th IFAC Symposium on Control in Transportation Systems Delft, The Netherlands, August 29-30-31, 2006
As here buses already can be freely moving after a14 along a bus lane, the IBL line has been designed to be implanted only between points a2 and a14. Along Page 239
the chosen lane separation line of the road surface, a series of IBL light boxes (with one LED array in each) have been installed, with about 3.5 meter separation between consecutive boxes. The single unit and a series of units along the surface are all shown in Fig. 2. Because of the long distance from point a2 to point a14, in order to flexibly manage the operations of all IBL signals, the whole IBL line in the demonstration site has been divided into five IBL sections: IBL11, from point a2 to point a4, with about 100 m length; IBL12, from a4 to a6, with about 93 m; IBL21, from a6 to a9, with about 100 m; IBL22, from a9 to a13, with about 98 m; IBL3, from a13 to a14, with about 98 m. The IBL light units located in the same IBL section all are connected together with the same cable, so that there is a total of five cables, therefore the lights in the same section can be all in “flashing” mode or all in “off” mode. Five cables have been separately connected with the IBL controller cabinet which is located at a7. It will decide the operations of the IBL lights on each section, based on the evaluation of the traffic situation and bus location, and drive the needed IBL lights. Then when the IBL signal units along a section are in “flashing” mode, the lane at the right side of the flashing lights is in a BUS lane status, and otherwise it is in the normal lane status, with all lights in “off” mode. Besides the IBL signals along the road surface, two other auxiliary traffic lights with the “Bus” symbol” are also located at a2 and a7, as shown in Fig. 3. If the “Bus” symbol is lighted “on”, the first IBL section located after that signal is in BUS lane status, with the corresponding IBL pavement lights “on”. The vertical light bus symbol is the legally binding signalling. This Variable Message Signal and the IBL signals along the pavement are operating together, which all are driven by the controller at a7. The use of both types of signals make it clearer for the drivers to know what is the current status of the rightmost lane in this area. In meantime, at the places a1 and a16, there are also two very large vertical static signs, as shown in Fig. 4. They intend to warn the coming drivers that they are about to be entering into the IBL area, thus raising their attention level. Along the IBL demonstration site, some loop detectors have been installed at locations a2, a3, a7, a8, a10 and a12, used to measure the vehicle flows and queue lengths, among which the detectors at a3 and a10 can also detect the vehicle speeds. At the locations a1, a6 and a11, there are also bus detectors installed, which can be activated by on-board transponders. But for the information of approach of buses still outside this demonstration area, GPS based data will be used. 3. ESTIMATIONS OF THE TRAFFIC FLOW © 11th IFAC Symposium on Control in Transportation Systems Delft, The Netherlands, August 29-30-31, 2006
As it has already been described in papers, (Viegas and Lu, 2001, 2004), the calculations and controls of all the related signals in an IBL system are closely dependent on the practical traffic situations and the bus movements within the chosen area. Here as some detectors have been installed, it is first to continuously estimate the needed information based on the locations and the types of detectors, and the detected data. 3.1. Estimation of flow rates at a2, a8 and a12. Some normal loop detectors have been installed at a2, a8 and a12, with loops being separately on every lane, or two loops at a2 and a8, and three loops at a12. Now with the number of detected vehicles passing over them within a given time period, the flow rates at these points can be easily estimated. Here every time, the calculations are based on the detected data only within last 240 s (4 minutes), or in fi,j=ni,j/240 (veh/s), where ni,j is units of vehicles passing over the loops at the jth lane of point i (a2, a8 or a12 ) within last 240 seconds. Then every 20 s, the estimation is updated. The total flow rate at one of point is the sum of ones on all lanes there. In practice, for loop detectors, whenever vehicles are moving very slowly through loops, there will be a large detecting error. Here if the queue length is less than the location of loops at a12, the estimated flow rates at a2, a8 and a12 are all useful. But whenever the end of the queue is found being accumulated between a8 and a12, only f8 and f2 are useful, without f12. Similarly whenever the queue length is located between a2 and a8, only f2 is useful. Moreover, if the end of the queue is beyond the location a2, the flow rates at these three points will all not be estimated, the controller directly use the queue length to determine the operations of IBL. 3.2. Estimation of vehicle speeds at a3 and a10. At a3 and a10, speed detectors are installed on every lane, and whenever a vehicle passes over any of them, its speed can be directly measured. But in practice, even for a same vehicle, during its movement, its speed at another point, say at a13, might be rather different from its speed at a3 or a10. In the meantime, because based on the principle of IBL operations, the determinations of IBL signals are made using data related to groups of vehicles, not individual vehicles. The average speed of all vehicles having passed over a3 or a10 within the last 240 s will be used, denoted as va3 and va10, by which we can know the speeds of the vehicles in the areas around a3 or a10. Some as in the case of estimating flow rates, a new estimation is made every 20 s. Similarly, whenever the queue length is longer than a10, the obtained va10 is useless, and speeds both at a3 and a10 are useless if the queue is longer than a2. 3.3. Estimation of queues
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Here as the distances of places a2, a3, a8, a10 and a12 to the stop line at intersection I1 are fixed, or in La2=588 m, La3=541 m, La8=390 m, La10=235 m, La12=185 m, whenever a queue is accumulated to one of them, its length can be easily estimated. But whenever the end of a queue is located between two points, its length can not be exactly known, therefore it has to be estimated on detected data. Similarly, a new estimation for the queue length, Lq, is going to be made every 20 s, with Lqnew=Lqold+5*20*(f-fout)/2, in which we suppose that vehicles at the queue occupy a length of about 5 meters. Lqold is the queue length in last step. fout is the average flow rate out of this arm at I1, and f is the traffic flow rates on two lanes at a2, a8 or a12, with f=f12, f=f8 or f=f2, based on the queue being located between a12 and I1, between a8 and a12, or between a2 and a8. If during the last 20 s, the end of the queue is around a2, a3, a8, a10 or a12, it will directly choose Lqnew=588 m, 541 m, 390 m, 235 m, or 185 m, without doing any other estimations. 3.4. Estimation of bus locations With the bus detectors installed at a1, a7 and a11, the coming buses then could be located individually. Now say Lb is the distance from the current location of a bus to the stop-line of the down-stream intersection I1. As the locations of a1, a7 and a11 are fixed, whenever a bus passes over a1, a7 or a11, it can be exactly known that Lb=588 m, Lb=395 m, or Lb=191 m. Similarly, whenever a bus is between a1 and a7, between a7 and a11, or after a11, its location has to be estimated. here a new estimation is made every 5 seconds, with Lbnew=Lbold-5*vb, in which Lbnew is the new location, Lbold is the location in last estimation, vb is the average bus speed on the historical data. Now suppose a bus has just passed through a1 at t=0. 5 seconds later, its estimated location will be in, Lbnew=Lbold-5*vb, in which Lbold=588 m. If further suppose vb=10 m/s, the estimated locations in the next steps will be in, Lbnew=538 m, 488 m, 438 m, 388 m…. But sometimes as the practical bus speed might be different from its historical speed or its estimations are different from its real locations, they will be often corrected by detected data. For instance, according to these estimations, at the end of the 4th step, the bus should have passed through the point a7. If a bus was detected at a11, it means this bus has really passed through a11, then at the detected moment of that bus, to set Lb=395 m, and confirm that bus no longer exists between a1 and a7. But if that bus has not been detected at a7 to the end of the 4th step, which means that bus still is located between a1 and a7, or its estimation is different from its real location. Here its estimation is corrected into © 11th IFAC Symposium on Control in Transportation Systems Delft, The Netherlands, August 29-30-31, 2006
Lbnew=405m. Although it might be not very accurate, but anyway it is sure this bus is still before a7. Similarly, according to its estimation at the end of the 3th step, bus has not passed through a7. But if it has been detected at a7, Lb will be in real data without making the 4th estimation. The estimations after a7 or a11 are going to be in same process, but with the first Lb in Lbold=395 m, or in Lbold=195 m according to its locations. Furthermore, if according to the estimations, Lbnew<0, we will say it has passed through the intersection, without checking whether it has really passed, as here that bus already can freely move on the bus lane between a14 and I1 without being influenced by any IBL signals. At a given moment, there might be more than one bus along the road from a1 to I1, the location of every bus is going to be estimated or corrected separately in the same way. Then from all Lb, it is easy to know how many buses are present, and their locations. Only after a bus has passed through a1, its locations can be detected or estimated through the data of bus detectors at a1, a7 and a11. But for the buses moving towards this IBL demonstration area, the data from the GPS system will be used, as in CARRIS most buses have been installed with GPS equipment, and their dynamic locations are reported with a frequency of 30 seconds. Then their estimations and corrections are same as the bus detectors at a1, a7 and a11. 4. OPERATION PROCESS OF THE SYSTEM As it is a big intersection at I1, the traffic signals are operating in multi-phase mode. During the peak hours, the cycle length is about 90 s, but the green phase time for this link is only 1/3 of cycle or in about 30 s, by which about (20+20) vehicles can pass through the intersection. In meantime, as at the intersection near a0, it is in two phase mode and vehicles might be coming in half cycle, it is often found that during peak hours the long queues are accumulated and some buses have to be involved in the queue. But in this chosen area, it also has its own special characteristics. For instance, besides the rather long distance, the number of lanes is not constant throughout the link. From a2 to a11, there are only two lanes. From a11 to a14, there are three lanes, in which the right one is used for the right turning vehicles, and from a14 to I1, it is then increased to four lanes, and one of them is the (permanent) bus lane. As there are not many right turning vehicles, the coming bus always can be more or less freely moving between a11 and I1 by first entering into the right-turning lane, then into the bus lane. In other words, only when the queue goes beyond a11, (or the queue length bigger than 200 m), the coming buses might be delayed, so the operations of IBL signals in this area will not be based on the whole queue, but just on the part after a11. Page 241
Because in peak hours, every cycle only about (20+20) vehicles can move out of the stop-line at I1, for the queue having accumulated to point a11 from I1, it needs at least two cycles to discharge them. So that the signals at I1 will directly influence the queue discharging between a14 and I1, indirectly influence the movements of vehicles in the queue between a11 and a14, and further less influences to vehicles after a11. In other words, to estimate the traffic situations around a11, it is not necessary to know the dynamics of every vehicle near I1, but only to how the number of vehicles having moved out of I1 in every cycle. In estimating the queue beyond point a11, we suppose that vehicles are always moving out of stop-line at I1 in average flow rate fout, without being further divided by in red times in which no vehicles move out and in green times, in which they move with the saturation flow rate. For instance, according to the obtained field data around I1 that every 90 s, about 40 vehicles moving out of the stop-line during 30 s green times, then no any vehicles out in the rest red times, we will say that here vehicles are moving out of stop-line in the average flow rate, fout=40/90 (veh/s), all the time, without further dividing it into f=40/30 (veh/s) during green times, and f=0 in red times. Therefore, in this way, the estimations of the queue length after point a11 and the calculations of IBL signals can be greatly simplified. According to results described in (Viegas and Lu, 2001), whenever a bus is found to be coming, basically, the IBL signals will determined based on Lb/vb+tstop-(n2/fc+tgdis), in which fc is is the saturation flow rate. Lb is the distance from the current location of the coming bus to the point a11, and totally there are (n2+n3) vehicles between them. Among them, n2 vehicles are freely moving, and n3 vehicles are in the queue. tgdis is queue discharging time, (only to the queue after point a11, not including the queue between a11 and I1). tstop is the time a bus spent at a bus-stop. Now if Lb/vb+tstop-(n2/fc+tgdis)<0, it is going to turn on IBL signals, otherwise there is no action on the IBL signals. After having analysed the practical data in this area, some simple data is found. When the queue is located between a10 and a12, to discharge this queue to less than a12, it needs about, 0
what it should do, otherwise, move to the next module. It will first enter into model 1. If i=20, 40, or 60, to calculate f2, f8 and f12. In other moments, just move to module 2, without doing any calculation. In module 2, if i=20, 40, or 60, to calculate v3, and v10. In other moments, just move to module 3, without doing any calculation. In module 3, if i=20, 40, or 60, to calculate queue length. In other moments, just move to module 4, without doing any calculation. In model 4, estimate bus locations from bus detectors every 5 s, and GPS message every 60 s. In module 5, it will decide to turn on or/and turn off the related IBL signals. The brief control process is that: when a bus is known to be moving towards this IBL demonstration area, according to the practical traffic situations which are estimated on detected data, to calculate the related control parameters, then some IBL signals might be turned on, before or after this bus arrives at a1. This bus will be more or less freely moving to the intersection I1. In the meantime, the IBL signals are turned off according to its locations and information on other buses. Say, when a bus passes through a4, IBL11 is always in “off” mode. Similarly, when it passes through a6, a9, a13, a14, IBL12, IBL21, IBL22 and IBL3 are in “off” mode in sequence. The processes to turn “on” and “off” IBL signals are going to be repeated in the same way. 5. SOME FIELD RESULTS 5.1. Impacts in Bus Movement Fig. 6 presents the comparison between the average bus speed in the first and second trimester of demonstration and the reference scenario (in the case of lines number 35, 38 and 68 the results are compared with the average reference scenario and with the average for that bus line before IBL installation). The results from this analysis reveal that the average bus speed increased after the installation of the IBL in all the lines that use this road link. The results point to average increases of 2,8 km/h in bus speed, which represent an improvement of some 20%. It is also important to point that the results in the second trimester of demonstration are considerably better than those of the first trimester, which means that the developments in the continuous improvement of the system succeed and also that the drivers acceptance of the system was good (particularly because the police enforcement of the system was concentrated in the first weeks of operation). Globally, the results of the evaluation procedure suggest a positive impact of the IBL in the bus speed Page 242
in the installation area, with an order of magnitude ranging from 5% to 20%. It is also important to underline the high consistency of these results, with the improvements being observed in all the bus lines using this road link and in the two methods of evaluation (continuous monitoring of bus movements and the comparison of the two weeks “special evaluation period”, in which all movements were filmed and then subject to measurement). 5.2. Impacts in General Traffic As it was previously referred the impacts in general traffic are difficult to evaluate, as the construction of a reliable reference scenario was not possible to undertake, but a continuous monitoring of traffic attributes was performed. The analysis of the traffic flows, vehicle speeds and queues accumulation during the demonstration period did not reveal any significant change in the overall pattern. This leads to the conclusion that the various modifications in the IBL control system didn’t create any visible impact in the general traffic movements. In addition, the comparison of the patterns for the various general traffic attributes during two weeks in March, one with and another without IBL activation gives no indication of significant changes. This leads to the conclusion that the IBL is not causing any significant impact in the general traffic main attributes.
an average of 5 minutes per hour in the last months of experience. Even during peak hours the IBL tend to be operating less than 25% of the time, which means the restrictions imposed to the general traffic are minimal. In short, the IBL is supposed to have a limited impact in road capacity as the restrictions to the flowing of the general traffic are, in maximum, less than 50% of the total road capacity and occur only for very limited periods of time. 6. CONCLUSIONS During the six months of demonstration several aspects were evaluated. Firstly, drivers reaction towards the system was good, having a good level of self-compliance. This was largely due to the understanding that exclusive allocation of the lane for buses was made strictly during the time needed for each bus. Second, the quantitative evaluation of the impact of the IBL on bus movement, general traffic and road capacity has shown promising results, namely: • •
•
5 to 20% increase in bus average speed in all routes that use this road link. No significant impact in general traffic main attributes (flow, vehicle speed and queues) was detected. The impact in road capacity was limited, as the IBL operates only for short time periods (up to 15 minutes per hour, during peak periods).
5.3. Restrictions to Road Capacity The installation of an IBL can result in a reduction of the overall capacity of a certain road link because, when the IBL is turned on, one of the lanes has a special status, during each is not accessible to general traffic. When the IBL in “on” the capacity of the road link available for the general traffic is reduced, and the level of that reduction is related with: (1) the number of lanes; (2) the percentage of vehicles that are authorized to use the IBL when it is “on”.
Considering these results the IBL seems to present a high potential to work as a tool to improve bus movement in cities. The results from the demonstration in Lisbon show that it can contribute to increase bus speed while causing a limited impact to the general traffic. Taking into account this rationale, it can be concluded that the IBL can be a very important mechanism to implement bus priority schemes in road section where bus frequency is low. REFERENCES
In the demonstration in Lisbon, the IBL was installed in a road section with two lanes, so the maximum reduction in road capacity would be 50%. But, a field study revealed that 5 to 6% of the vehicles using this road section are taxes and buses, which are authorized to use the IBL when activated (one should also consider that vehicles turning right in a crossing are also authorized to use the IBL). Considering this, when the IBL is on the reduction in road capacity will always be less than 50%, in other words, the level of saturation of the remaining lane won’t double. Even though the theoretical analysis revealed that the impact f the IBL in road capacity would be less than 50%, the main difference of the IBL to a regular bus lane is the fact that the restrictions are imposed only within limited periods. The data from the demonstration in Lisbon suggests that, in average, the IBL is activated up to 15 minutes per hour, with © 11th IFAC Symposium on Control in Transportation Systems Delft, The Netherlands, August 29-30-31, 2006
Viegas, J., “Turn of the century, survival of the compact city, revival of public transport.Bottlenecks”, in Transportation and the Port Industry. (H. Meersman, Ed). Antwerp, Belgium, 1996. Viegas J. and Lu B., "Widening the scope for bus priority with intermittent bus lanes", Transportation Planning and Technology Vol.24, pp87-110, 2001 Viegas J. and Lu B., "The intermittent bus lane signal settings within an area", Transportation Research Part C 12 (2004) 453-469
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Fig. 1 Basic Data i=0 i=i+1 Estimation of traffic flow Estimation of vehicle speed Fig. 2 Estimation of queue length Estimation of bus locations Decisions and operations of IBL i=60
Fig. 5 Fig. 3 IBL Demonstration 1st Trimester
IBL Demonstration 2nd Trimester
IBL Demonstration Total
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© 11th IFAC Symposium on Control in Transportation Systems Delft, The Netherlands, August 29-30-31, 2006
Line nu. 35
Line nu. 38
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Av erage
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Abstract: This paper introduces the demonstration of a new bus priority measure, the Intermittent Bus Lane, in Lisbon. The situations around the chosen testing area, the arrangements and installations of the related signals, signs and other system equipment all have been shown. The detections and estimations of traffic flow and bus movements, and the control process are also described. Finally some analysis on the field data about the obtained bus priority and the influences on the general traffic flow are also given. Copyright © 2006 IFAC Keywords: Public Transport, Integrated Traffic Control, Detector, Bus Priority, Estimation.
1. INTRODUCTION
2. BASIC DESCRIPTIONS OF THE SYSTEM
Intermittent Bus Lane (IBL) is a new bus priority measure. Its concept was first introduced by Viegas, (1996), then it has been further developed theoretically (Viegas and Lu, 2001, 2004). In these papers, its concept, structure, operations, the movements of buses and other vehicles on IBL, modelling, and optimal control equations etc. all have been described in details.
After having investigated many possible areas for the IBL demonstration in Lisbon, including the geometry, the real traffic flow situations and bus movements, seven favourite places for IBL application were identified. Finally, after discussion with the Municipality and Carris, the site at the Alameda da Universidade was selected as the first IBL demonstration area.
Normally since theoretical research can not include all possible practical situations, a real world application is necessary to act as a prototype. In order to do that, the Demonstration project of the IBL in Lisbon has been launched, in which the Municipality of Lisbon, the urban bus operator (Carris) and the School of Engineering of the Technical University of Lisbon (Instituto Superior Técnico) are all involved. This paper presents the main aspects of this demonstration project: the choice of the demonstration area; the arrangements of the related signals, signs, detectors and further system equipment within the chosen area; the detections and estimations of traffic flow and bus movements, and the control process; finally some analysis on the field data about the obtained bus priority and the influences on general traffic flow.
The map around this demonstration area is shown as in Fig. 1. At I1, a very big intersection is located. From point a0 to the stop-line of I1 a relatively long (800 m) stretch of one way road exists, with two straight alignments connected by an “S” curve roughly at the middle. Along the stretch, between a1 and a13, there are two lanes. From a13 to a14 there are three lanes where one lane can be used only by the right-turn moving vehicles, then from a14 to I1, there are four lanes, and one of them is the (permanent) Bus lane. Normal traffic signal lights are operating at I1, but from a0 to I1, there are no other traffic lights. At b1 and b2 two bus stops are located. In total four bus lines pass through these two bus stops, with a frequency of about 18,2 bus/h.
© 11th IFAC Symposium on Control in Transportation Systems Delft, The Netherlands, August 29-30-31, 2006
As here buses already can be freely moving after a14 along a bus lane, the IBL line has been designed to be implanted only between points a2 and a14. Along Page 239
the chosen lane separation line of the road surface, a series of IBL light boxes (with one LED array in each) have been installed, with about 3.5 meter separation between consecutive boxes. The single unit and a series of units along the surface are all shown in Fig. 2. Because of the long distance from point a2 to point a14, in order to flexibly manage the operations of all IBL signals, the whole IBL line in the demonstration site has been divided into five IBL sections: IBL11, from point a2 to point a4, with about 100 m length; IBL12, from a4 to a6, with about 93 m; IBL21, from a6 to a9, with about 100 m; IBL22, from a9 to a13, with about 98 m; IBL3, from a13 to a14, with about 98 m. The IBL light units located in the same IBL section all are connected together with the same cable, so that there is a total of five cables, therefore the lights in the same section can be all in “flashing” mode or all in “off” mode. Five cables have been separately connected with the IBL controller cabinet which is located at a7. It will decide the operations of the IBL lights on each section, based on the evaluation of the traffic situation and bus location, and drive the needed IBL lights. Then when the IBL signal units along a section are in “flashing” mode, the lane at the right side of the flashing lights is in a BUS lane status, and otherwise it is in the normal lane status, with all lights in “off” mode. Besides the IBL signals along the road surface, two other auxiliary traffic lights with the “Bus” symbol” are also located at a2 and a7, as shown in Fig. 3. If the “Bus” symbol is lighted “on”, the first IBL section located after that signal is in BUS lane status, with the corresponding IBL pavement lights “on”. The vertical light bus symbol is the legally binding signalling. This Variable Message Signal and the IBL signals along the pavement are operating together, which all are driven by the controller at a7. The use of both types of signals make it clearer for the drivers to know what is the current status of the rightmost lane in this area. In meantime, at the places a1 and a16, there are also two very large vertical static signs, as shown in Fig. 4. They intend to warn the coming drivers that they are about to be entering into the IBL area, thus raising their attention level. Along the IBL demonstration site, some loop detectors have been installed at locations a2, a3, a7, a8, a10 and a12, used to measure the vehicle flows and queue lengths, among which the detectors at a3 and a10 can also detect the vehicle speeds. At the locations a1, a6 and a11, there are also bus detectors installed, which can be activated by on-board transponders. But for the information of approach of buses still outside this demonstration area, GPS based data will be used. 3. ESTIMATIONS OF THE TRAFFIC FLOW © 11th IFAC Symposium on Control in Transportation Systems Delft, The Netherlands, August 29-30-31, 2006
As it has already been described in papers, (Viegas and Lu, 2001, 2004), the calculations and controls of all the related signals in an IBL system are closely dependent on the practical traffic situations and the bus movements within the chosen area. Here as some detectors have been installed, it is first to continuously estimate the needed information based on the locations and the types of detectors, and the detected data. 3.1. Estimation of flow rates at a2, a8 and a12. Some normal loop detectors have been installed at a2, a8 and a12, with loops being separately on every lane, or two loops at a2 and a8, and three loops at a12. Now with the number of detected vehicles passing over them within a given time period, the flow rates at these points can be easily estimated. Here every time, the calculations are based on the detected data only within last 240 s (4 minutes), or in fi,j=ni,j/240 (veh/s), where ni,j is units of vehicles passing over the loops at the jth lane of point i (a2, a8 or a12 ) within last 240 seconds. Then every 20 s, the estimation is updated. The total flow rate at one of point is the sum of ones on all lanes there. In practice, for loop detectors, whenever vehicles are moving very slowly through loops, there will be a large detecting error. Here if the queue length is less than the location of loops at a12, the estimated flow rates at a2, a8 and a12 are all useful. But whenever the end of the queue is found being accumulated between a8 and a12, only f8 and f2 are useful, without f12. Similarly whenever the queue length is located between a2 and a8, only f2 is useful. Moreover, if the end of the queue is beyond the location a2, the flow rates at these three points will all not be estimated, the controller directly use the queue length to determine the operations of IBL. 3.2. Estimation of vehicle speeds at a3 and a10. At a3 and a10, speed detectors are installed on every lane, and whenever a vehicle passes over any of them, its speed can be directly measured. But in practice, even for a same vehicle, during its movement, its speed at another point, say at a13, might be rather different from its speed at a3 or a10. In the meantime, because based on the principle of IBL operations, the determinations of IBL signals are made using data related to groups of vehicles, not individual vehicles. The average speed of all vehicles having passed over a3 or a10 within the last 240 s will be used, denoted as va3 and va10, by which we can know the speeds of the vehicles in the areas around a3 or a10. Some as in the case of estimating flow rates, a new estimation is made every 20 s. Similarly, whenever the queue length is longer than a10, the obtained va10 is useless, and speeds both at a3 and a10 are useless if the queue is longer than a2. 3.3. Estimation of queues
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Here as the distances of places a2, a3, a8, a10 and a12 to the stop line at intersection I1 are fixed, or in La2=588 m, La3=541 m, La8=390 m, La10=235 m, La12=185 m, whenever a queue is accumulated to one of them, its length can be easily estimated. But whenever the end of a queue is located between two points, its length can not be exactly known, therefore it has to be estimated on detected data. Similarly, a new estimation for the queue length, Lq, is going to be made every 20 s, with Lqnew=Lqold+5*20*(f-fout)/2, in which we suppose that vehicles at the queue occupy a length of about 5 meters. Lqold is the queue length in last step. fout is the average flow rate out of this arm at I1, and f is the traffic flow rates on two lanes at a2, a8 or a12, with f=f12, f=f8 or f=f2, based on the queue being located between a12 and I1, between a8 and a12, or between a2 and a8. If during the last 20 s, the end of the queue is around a2, a3, a8, a10 or a12, it will directly choose Lqnew=588 m, 541 m, 390 m, 235 m, or 185 m, without doing any other estimations. 3.4. Estimation of bus locations With the bus detectors installed at a1, a7 and a11, the coming buses then could be located individually. Now say Lb is the distance from the current location of a bus to the stop-line of the down-stream intersection I1. As the locations of a1, a7 and a11 are fixed, whenever a bus passes over a1, a7 or a11, it can be exactly known that Lb=588 m, Lb=395 m, or Lb=191 m. Similarly, whenever a bus is between a1 and a7, between a7 and a11, or after a11, its location has to be estimated. here a new estimation is made every 5 seconds, with Lbnew=Lbold-5*vb, in which Lbnew is the new location, Lbold is the location in last estimation, vb is the average bus speed on the historical data. Now suppose a bus has just passed through a1 at t=0. 5 seconds later, its estimated location will be in, Lbnew=Lbold-5*vb, in which Lbold=588 m. If further suppose vb=10 m/s, the estimated locations in the next steps will be in, Lbnew=538 m, 488 m, 438 m, 388 m…. But sometimes as the practical bus speed might be different from its historical speed or its estimations are different from its real locations, they will be often corrected by detected data. For instance, according to these estimations, at the end of the 4th step, the bus should have passed through the point a7. If a bus was detected at a11, it means this bus has really passed through a11, then at the detected moment of that bus, to set Lb=395 m, and confirm that bus no longer exists between a1 and a7. But if that bus has not been detected at a7 to the end of the 4th step, which means that bus still is located between a1 and a7, or its estimation is different from its real location. Here its estimation is corrected into © 11th IFAC Symposium on Control in Transportation Systems Delft, The Netherlands, August 29-30-31, 2006
Lbnew=405m. Although it might be not very accurate, but anyway it is sure this bus is still before a7. Similarly, according to its estimation at the end of the 3th step, bus has not passed through a7. But if it has been detected at a7, Lb will be in real data without making the 4th estimation. The estimations after a7 or a11 are going to be in same process, but with the first Lb in Lbold=395 m, or in Lbold=195 m according to its locations. Furthermore, if according to the estimations, Lbnew<0, we will say it has passed through the intersection, without checking whether it has really passed, as here that bus already can freely move on the bus lane between a14 and I1 without being influenced by any IBL signals. At a given moment, there might be more than one bus along the road from a1 to I1, the location of every bus is going to be estimated or corrected separately in the same way. Then from all Lb, it is easy to know how many buses are present, and their locations. Only after a bus has passed through a1, its locations can be detected or estimated through the data of bus detectors at a1, a7 and a11. But for the buses moving towards this IBL demonstration area, the data from the GPS system will be used, as in CARRIS most buses have been installed with GPS equipment, and their dynamic locations are reported with a frequency of 30 seconds. Then their estimations and corrections are same as the bus detectors at a1, a7 and a11. 4. OPERATION PROCESS OF THE SYSTEM As it is a big intersection at I1, the traffic signals are operating in multi-phase mode. During the peak hours, the cycle length is about 90 s, but the green phase time for this link is only 1/3 of cycle or in about 30 s, by which about (20+20) vehicles can pass through the intersection. In meantime, as at the intersection near a0, it is in two phase mode and vehicles might be coming in half cycle, it is often found that during peak hours the long queues are accumulated and some buses have to be involved in the queue. But in this chosen area, it also has its own special characteristics. For instance, besides the rather long distance, the number of lanes is not constant throughout the link. From a2 to a11, there are only two lanes. From a11 to a14, there are three lanes, in which the right one is used for the right turning vehicles, and from a14 to I1, it is then increased to four lanes, and one of them is the (permanent) bus lane. As there are not many right turning vehicles, the coming bus always can be more or less freely moving between a11 and I1 by first entering into the right-turning lane, then into the bus lane. In other words, only when the queue goes beyond a11, (or the queue length bigger than 200 m), the coming buses might be delayed, so the operations of IBL signals in this area will not be based on the whole queue, but just on the part after a11. Page 241
Because in peak hours, every cycle only about (20+20) vehicles can move out of the stop-line at I1, for the queue having accumulated to point a11 from I1, it needs at least two cycles to discharge them. So that the signals at I1 will directly influence the queue discharging between a14 and I1, indirectly influence the movements of vehicles in the queue between a11 and a14, and further less influences to vehicles after a11. In other words, to estimate the traffic situations around a11, it is not necessary to know the dynamics of every vehicle near I1, but only to how the number of vehicles having moved out of I1 in every cycle. In estimating the queue beyond point a11, we suppose that vehicles are always moving out of stop-line at I1 in average flow rate fout, without being further divided by in red times in which no vehicles move out and in green times, in which they move with the saturation flow rate. For instance, according to the obtained field data around I1 that every 90 s, about 40 vehicles moving out of the stop-line during 30 s green times, then no any vehicles out in the rest red times, we will say that here vehicles are moving out of stop-line in the average flow rate, fout=40/90 (veh/s), all the time, without further dividing it into f=40/30 (veh/s) during green times, and f=0 in red times. Therefore, in this way, the estimations of the queue length after point a11 and the calculations of IBL signals can be greatly simplified. According to results described in (Viegas and Lu, 2001), whenever a bus is found to be coming, basically, the IBL signals will determined based on Lb/vb+tstop-(n2/fc+tgdis), in which fc is is the saturation flow rate. Lb is the distance from the current location of the coming bus to the point a11, and totally there are (n2+n3) vehicles between them. Among them, n2 vehicles are freely moving, and n3 vehicles are in the queue. tgdis is queue discharging time, (only to the queue after point a11, not including the queue between a11 and I1). tstop is the time a bus spent at a bus-stop. Now if Lb/vb+tstop-(n2/fc+tgdis)<0, it is going to turn on IBL signals, otherwise there is no action on the IBL signals. After having analysed the practical data in this area, some simple data is found. When the queue is located between a10 and a12, to discharge this queue to less than a12, it needs about, 0
what it should do, otherwise, move to the next module. It will first enter into model 1. If i=20, 40, or 60, to calculate f2, f8 and f12. In other moments, just move to module 2, without doing any calculation. In module 2, if i=20, 40, or 60, to calculate v3, and v10. In other moments, just move to module 3, without doing any calculation. In module 3, if i=20, 40, or 60, to calculate queue length. In other moments, just move to module 4, without doing any calculation. In model 4, estimate bus locations from bus detectors every 5 s, and GPS message every 60 s. In module 5, it will decide to turn on or/and turn off the related IBL signals. The brief control process is that: when a bus is known to be moving towards this IBL demonstration area, according to the practical traffic situations which are estimated on detected data, to calculate the related control parameters, then some IBL signals might be turned on, before or after this bus arrives at a1. This bus will be more or less freely moving to the intersection I1. In the meantime, the IBL signals are turned off according to its locations and information on other buses. Say, when a bus passes through a4, IBL11 is always in “off” mode. Similarly, when it passes through a6, a9, a13, a14, IBL12, IBL21, IBL22 and IBL3 are in “off” mode in sequence. The processes to turn “on” and “off” IBL signals are going to be repeated in the same way. 5. SOME FIELD RESULTS 5.1. Impacts in Bus Movement Fig. 6 presents the comparison between the average bus speed in the first and second trimester of demonstration and the reference scenario (in the case of lines number 35, 38 and 68 the results are compared with the average reference scenario and with the average for that bus line before IBL installation). The results from this analysis reveal that the average bus speed increased after the installation of the IBL in all the lines that use this road link. The results point to average increases of 2,8 km/h in bus speed, which represent an improvement of some 20%. It is also important to point that the results in the second trimester of demonstration are considerably better than those of the first trimester, which means that the developments in the continuous improvement of the system succeed and also that the drivers acceptance of the system was good (particularly because the police enforcement of the system was concentrated in the first weeks of operation). Globally, the results of the evaluation procedure suggest a positive impact of the IBL in the bus speed Page 242
in the installation area, with an order of magnitude ranging from 5% to 20%. It is also important to underline the high consistency of these results, with the improvements being observed in all the bus lines using this road link and in the two methods of evaluation (continuous monitoring of bus movements and the comparison of the two weeks “special evaluation period”, in which all movements were filmed and then subject to measurement). 5.2. Impacts in General Traffic As it was previously referred the impacts in general traffic are difficult to evaluate, as the construction of a reliable reference scenario was not possible to undertake, but a continuous monitoring of traffic attributes was performed. The analysis of the traffic flows, vehicle speeds and queues accumulation during the demonstration period did not reveal any significant change in the overall pattern. This leads to the conclusion that the various modifications in the IBL control system didn’t create any visible impact in the general traffic movements. In addition, the comparison of the patterns for the various general traffic attributes during two weeks in March, one with and another without IBL activation gives no indication of significant changes. This leads to the conclusion that the IBL is not causing any significant impact in the general traffic main attributes.
an average of 5 minutes per hour in the last months of experience. Even during peak hours the IBL tend to be operating less than 25% of the time, which means the restrictions imposed to the general traffic are minimal. In short, the IBL is supposed to have a limited impact in road capacity as the restrictions to the flowing of the general traffic are, in maximum, less than 50% of the total road capacity and occur only for very limited periods of time. 6. CONCLUSIONS During the six months of demonstration several aspects were evaluated. Firstly, drivers reaction towards the system was good, having a good level of self-compliance. This was largely due to the understanding that exclusive allocation of the lane for buses was made strictly during the time needed for each bus. Second, the quantitative evaluation of the impact of the IBL on bus movement, general traffic and road capacity has shown promising results, namely: • •
•
5 to 20% increase in bus average speed in all routes that use this road link. No significant impact in general traffic main attributes (flow, vehicle speed and queues) was detected. The impact in road capacity was limited, as the IBL operates only for short time periods (up to 15 minutes per hour, during peak periods).
5.3. Restrictions to Road Capacity The installation of an IBL can result in a reduction of the overall capacity of a certain road link because, when the IBL is turned on, one of the lanes has a special status, during each is not accessible to general traffic. When the IBL in “on” the capacity of the road link available for the general traffic is reduced, and the level of that reduction is related with: (1) the number of lanes; (2) the percentage of vehicles that are authorized to use the IBL when it is “on”.
Considering these results the IBL seems to present a high potential to work as a tool to improve bus movement in cities. The results from the demonstration in Lisbon show that it can contribute to increase bus speed while causing a limited impact to the general traffic. Taking into account this rationale, it can be concluded that the IBL can be a very important mechanism to implement bus priority schemes in road section where bus frequency is low. REFERENCES
In the demonstration in Lisbon, the IBL was installed in a road section with two lanes, so the maximum reduction in road capacity would be 50%. But, a field study revealed that 5 to 6% of the vehicles using this road section are taxes and buses, which are authorized to use the IBL when activated (one should also consider that vehicles turning right in a crossing are also authorized to use the IBL). Considering this, when the IBL is on the reduction in road capacity will always be less than 50%, in other words, the level of saturation of the remaining lane won’t double. Even though the theoretical analysis revealed that the impact f the IBL in road capacity would be less than 50%, the main difference of the IBL to a regular bus lane is the fact that the restrictions are imposed only within limited periods. The data from the demonstration in Lisbon suggests that, in average, the IBL is activated up to 15 minutes per hour, with © 11th IFAC Symposium on Control in Transportation Systems Delft, The Netherlands, August 29-30-31, 2006
Viegas, J., “Turn of the century, survival of the compact city, revival of public transport.Bottlenecks”, in Transportation and the Port Industry. (H. Meersman, Ed). Antwerp, Belgium, 1996. Viegas J. and Lu B., "Widening the scope for bus priority with intermittent bus lanes", Transportation Planning and Technology Vol.24, pp87-110, 2001 Viegas J. and Lu B., "The intermittent bus lane signal settings within an area", Transportation Research Part C 12 (2004) 453-469
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Fig. 1 Basic Data i=0 i=i+1 Estimation of traffic flow Estimation of vehicle speed Fig. 2 Estimation of queue length Estimation of bus locations Decisions and operations of IBL i=60
Fig. 5 Fig. 3 IBL Demonstration 1st Trimester
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