Australasian Transport Research Forum 2015 Proceedings 30 September - 2 October 2015, Sydney, Australia Publication website: http://www.atrf.info/papers/index.aspx A prospective way of ramp metering using vehicle infrastructure integration system Tuo Mao1, Vinayak Dixit2 1Department of Civil and Environmental Engineering, The University of New South Wales, Australia Email: [email protected] 2Department of Civil and Environmental Engineering, The University of New South Wales, Australia Email: [email protected] Abstract As Bluetooth and navigation devices are widely used on latest produced vehicles, vehicle infrastructure integration (VII) system in Intelligent Transport System (ITS) has been given flourishing expectation because of its detection accuracy and multiple-information collecting capability. This paper suggests an ideal ITS environment where all vehicles are equipped with certain VII device and all vehicles can be recognized by ITS which is capable of memorizing all vehicles’ route choices. This paper proposed an innovative hypothetical theory and algorithm of coordinated ramp metering which takes the advantage of VII system. This algorithm integrates vehicle routing information with coordinated ramp metering by reading and processing the vehicles’ average travel distance along the freeway. Comparison between new coordinated ramp metering algorithm and traditional local ramp metering strategy has been made and travel time saving is observed in new ramp metering algorithm. 1. Introduction 1.1 Vehicle infrastructure integration Vehicle infrastructure integration is defined as the establishment of vehicle-to-vehicle and vehicle-to-roadside communication capability by Paniati (2005). Potential addressed in mobility, safety, and commercial benefits are clear. From hardware aspect, the proof of concept (POC) test of vehicle infrastructure integration was performed by the US department of transportation in the northwest suburbs of Detroit, Michigan in Kandarpa et al. (2009)’s report. This report constructs a structure of vehicle onboard equipment (OBE) and roadside equipment (RSE). OBE consists a dedicated short-range communications (DSRC) radio, a highly accurate on-board positioning system, an appropriately configured on-board computer. RSE would be linked to the specialized VII network. Communication between OBE and RSE, as well as between different OBEs, takes the advantage of DSRC radio. 400 meters is the maximum communication range between OBE and RSE is found in this test. Information passed by VII system referred to mobility, safety and commercial applications. Mobility information contains position data and time label and the accuracy of position data is 4.5 m at 95% circular error probable. It is also worth mentioning that VII system has the capability of receive vehicles’ origin & destination (OD) and suggest vehicle’s route information within the network enabler service. No traffic management and transportation efficiency application and benefit analysis were performed during the POC test. From the corresponding software aspect, effort has been made into traffic condition monitoring (Ma et al., 2009, Li et al., 2008), collision avoidance (Wang et al., 2011, Chen and Cai, 2005), and weather-related application (Petty and Mahoney III, 2007). Applications in traffic condition monitoring and collision avoidance take advantage of real-time position data to better understand traffic condition. In addition, weather-related application makes use of probe 1 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) vehicle data to diagnose and predict road weather condition in order to reduce weather-related accidents. Few researches are referred to traffic management and control taking the advantage of VII. Lee and Park (2008) Create a microscopic simulation test bed under the cooperative vehicleinfrastructure environment to evaluate route guidance application. The route guidance strategy uses the average link travel times collected from probe vehicles through VII system. Later, Park and Lee (2009) evaluate sustainability impact of route guidance system based on the same test bed under cooperative vehicle-infrastructure environment. Algorithm of variable speed limit (VSL) using the data provided by VII system is developed by Sun et al. (2014). Information such as vehicle models, vehicles’ speed and position are collected by VII system and utilized by VSL to keep the traffic flow at a high efficiency and suppress propagation of shockwaves. Above all, the VII system is mainly utilized for its real-time mobility data such as vehicle’s velocity and position. No research has been made to research the other information such as route information. The research gap exists in processing of the unused information and make the most of VII system. 1.2 Ramp metering A ramp meter, ramp signal or metering light is a device, usually a basic traffic light or a twosection signal (red and green only, no yellow) light together with a signal controller that regulates the flow of traffic entering freeways according to current traffic conditions. It is the use of traffic signals at freeway on-ramps to manage the rate of automobiles entering the freeway. Ramp metering systems have proved to be successful in decreasing traffic congestion and improving driving safety. Ramp meters are claimed to reduce congestion (increase speed and volume) on freeways by reducing demand and by breaking up platoons of cars. Two variations of demand reduction are commonly cited; one being access rate, the other diversion (Piotrowicz and Robinson, 1995). Ramp metering can be divided into local ramp metering and coordinated ramp metering. ALINEA is a typical control strategy based on classical feedback theory which keeps the mainline occupancy stay nearing a desired value by adjusting the corresponding ramp metering ratio (Papageorgiou et al., 1991, Hou et al., 2011, Demiral and Celikoglu, 2011, Abdel-Aty et al., 2007). FLOW, another local ramp metering algorithm, is an integrated, trafficresponsive metering algorithm which is determined by local conditions (Local Metering Rate or LMR) or system capacity (Bottleneck Metering Rate or BMR) or on-ramp queue length (Hasan et al., 2002). HERO (heuristic ramp-metering coordination) can be classified into coordinated ramp metering because it is extended from ALINEA by adding a heuristic coordination scheme which overcomes the uncertainty of freeway capacity by identifying the critical downstream occupancy for maximum throughput (Papamichail et al., 2010). Kotsialos and Papageorgiou (2004) applied nonlinear discrete-time optimal control to ramp metering by creating a macroscopic traffic flow model (METANET model) which divides a motorway link into several sections with its on-ramps and off-ramps. By discretization of freeway, dynamic real time simulation can be seen as a sum of several time intervals. During each interval, each section of freeway contains its own time-dependent status vector and the status vector is also determined by the control vector which represents ramp metering’s influence. No-linear optimization was selected to solve the discrete-time model in the report. The macroscopic traffic flow model connects coordinated signal control and the non-linear optimal control which not only makes a system-wide optimization but also classifies a new approach of ramp metering optimization (Hasan et al., 2002). 2 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) 1.3 Overview All ramp metering methodologies take count in traditional loop detector data and none of them uses data from VII system. No existing ramp metering algorithm has the capability of understanding traffic demand, in other word, origin destination information have not been utilized due to the lack of VII system broadcasting. However, VII system’s capability of receiving routing requests and provide route guidance makes it possible for traffic engineer to bypass traffic demand estimation and traffic assignment model and, instead, to read driver’s origin-destination and route choice in real-time through VII system. For this point of view, this paper describe an OD understanding ramp metering algorithm. The rest of this paper is organized as follow. 2. Methodology This section discusses VII system environment specification, the assumptions made, Route information digitization, the theory development that guiding heuristic ramp metering strategy and the innovative rules within ramp metering strategy. 2.1 VII system environment specification To simplify the case, an ideal VII system environment is assumed. In this ideal environment, all vehicles are equipped with standard on-board equipment and every entry ramp of the research motorway are equipped with standard roadside equipment. The maximum reliable communication distance between OBE and RSE is set to 400 meters. Communicative information consists of vehicle position, time label, vehicle identification and vehicle speed. 2.2 Route information digitization Since it is capable to track and read vehicle’s route information using VII system, the collected data must be recorded and processed in a relatively easy way. A solution come out of this paper is to transfer the origin-destination information into one number, travel distance along the investigated motorway. The travel distance along the motorway for vehicle #i is𝑙𝑖 . The reasons using travel distance along the investigated motorway 𝑙𝑖 are (1) it turns a two-field (origin and destination) format into one-field format; (2) it transfers the denotation of origin and destination information into a quantified value for the ease of further decision making; and (3) it is easy to process into a link-wide variable by taking average which will be discussed in the following sections. 2.3 Existing ramp metering framework The HERO algorithm built by Papamichail et al. (2010) can be divided into two level. The lower level uses ALINEA (Papageorgiou et al., 1991), a local feedback ramp metering strategy. The higher level applies a feed-forward disturbance compensation (FDC) strategy. FDC is a known control theory methodology that enhances the robustness of a feedback controller such as the ALINEA. The hybrid of two levels is a feedback feed-forward control scheme. The following sections describe the local feedback ramp metering strategy and the FDC strategy respectively. 2.3.1. ALINEA – local feedback ramp metering strategy The local control unit in ALINEA comprises a single metered ramp that regulates the metering rate r by adapting to traffic conditions monitored by one or several mainline detector stations. The traffic conditions are namely the lane occupancy as measured by vehicle detectors. 3 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) For each detector station on the mainline of the motorway, a critical occupancy denoted as o crit is calibrated based on the flow-occupancy diagram showed in Figure 1. Figure 1 Flow vs. occupancy diagram At time-step k, for each detector j, the difference between measured occupancy from mainline detector 𝑜𝑚𝑒𝑎𝑠,𝑗 (𝑘) and its critical occupancy 𝑜𝑐𝑟𝑖𝑡,𝑗 is denoted as 𝑑𝑗 (𝑘) which is shown in Eq. ⑴. 𝑑𝑗 (𝑘) = 𝑜𝑐𝑟𝑖𝑡,𝑗 − o𝑚𝑒𝑎𝑠,𝑗 (k) ⑴ An indicator 𝛼𝑗 (𝑘) is introduced to represent the inverse trend of on-ramp flow expectation from lower level ramp metering in time step k. The difference 𝑑𝑗 (𝑘) is accumulated into indicator 𝛼𝑗 (𝑘) by the way shown in Eq.⑵. 𝛼𝑗 (𝑘) = 𝛼𝑗 (𝑘 − 1) + 𝑐(𝑑𝑗 (𝑘)) ⑵ Where 𝑐(𝑑𝑗 (𝑘)) is an alpha-increment function which is shown in Figure 2. Figure 2 Alpha increment function The relationship between 𝛼𝑗 (𝑘) and according ramp metering ratio 𝑟𝑠 (𝑘) is shown in Eq.⑶. 4 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) ⑶ 𝑟𝑠 (𝑘) = (1 − 𝛼𝑗 (𝑘)) × 𝑞𝑠 (k − 1)/𝑑𝑠 (𝑘) where 𝑞𝑠 (k − 1) is the on-ramp flow in (k-1) time step. 𝑑𝑠 (𝑘) is the demand of the on-ramp at time step k, which is detected from upstream RSE or loop detector station. In time step k, the expected flow on ramp meter s is deducted by Eq. ⑷. ⑷ 𝑞𝑠 (k) = (1 − 𝛼𝑗 (𝑘)) × 𝑑𝑠 (𝑘) 2.3.2. FDC strategy The FDC is implemented by re-using a known ramp metering strategy called the DemandCapacity strategy. The FDC starts by estimating the motorway spare capacity N as a function of the upstream arrivals A (the disturbance) and downstream nominal bottleneck capacity B. N=B−A ⑸ To apply an effective disturbance compensation strategy, the locations of the capacity B and the station measuring A must be configured by the user such that the distance between them desirably does not exceed 2km with a maximum of 3km. In practice, between locations A and B (the boundaries of the coordinated freeway section), see Figure 3 for an example, any of the following could exist: (1) Multiple metered ramps (𝑄1 , 𝑄2 , 𝑄3 ) (2) Unmetered on-ramps (U) (3) Off-ramps (X). Figure 3 Freeway coordinated section For scenarios similar to Figure 3, Eq. ⑸ is generalised as N = B−A−U+X ⑹ where U is the sum of the flows of all unmetered ramps (measured by detectors) in the section and X is the sum of the flows of all off-ramps in the zone (measured by detectors). Note that the section concept in the FDC strategy is not to be confused with the cluster of the ALINEA strategy. The section is a local aggregation of ramps that can share the same disturbance measurements. The dynamic cluster of the ALINEA is a wide-area coordination strategy. To apply FDC to the ALINEA strategy, a compensation parameter β is introduced to fit the demand-capacity strategy, so Eq. ⑷ is modified to 𝑞𝑠 (k) = (1 − 𝛼𝑗 (𝑘)) (1 + 𝛽) × 𝑑𝑠 (𝑘) ⑺ where 5 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) β= N−∑𝑠∈𝑆 𝑑𝑠 (k) ∑𝑠∈𝑆 𝑑𝑠 (k) ⑻ and where ∑𝑠∈𝑆 𝑑𝑠 (k) is the sum of target flows of each metered ramps s in the section. These flows are configured as estimates of the expected ramp demands. As a result, flow N is shared between the metered ramps in proportion to their target flows 𝑑𝑠 (k) which are the expected unconstrained demands on each ramp. Similarly to 𝛼𝑗 (𝑘) , the 𝛽 parameter gives an indication of the mainline spare (or lack of) capacity. If β > 0, it follows that N > ∑𝑠∈𝑆 𝑑𝑠 (k) indicates spare capacity is available or spare capacity is on the rise. If β < 0, it follows that N < ∑𝑠∈𝑆 𝑑𝑠 (k) indicates demand is larger than capacity or demand is on the rise. 2.4 Extension A hypothesis is suggested in this experiment that, for all on-ramps, the shorter average distance vehicles travel along freeway the higher freeway output (off-ramp) flow, then more drivers can be served in certain amount of time (especially rush hours). After this hypothesis, heuristic rules that prefer drivers that travel a shorter distance along freeway is extended in the HERO strategy. 2.4.1. Hypothesis The hypothesis proposed is that, for all on-ramps, the shorter average distance vehicles travel along the higher freeway output (off-ramp) flow, then more drivers can be served in certain amount of time (especially rush hours). Ideally, shorter travel distance means getting off the freeway sooner, and less downstream bottleneck experienced and caused, more vacancy for the down-steam on-ramps, and in the end, more efficient the freeway services drivers. 2.4.2. Fitting in HERO strategy In the hypothesis, preference for vehicles travel shorter distance along freeway is suggested. Since for all on-ramps, a first-in-first-out (FIFO) rule must be obeyed, it is unrealistic to implicate the preference for those preferred vehicles individually. Instead, a fuzzy way is proposed that preference can be allocated onto each on-ramp by judging its average distance vehicles travel along freeway. As the definition of average travel distance at on-ramp s on the freeway at time step k, 𝐿̅𝑠 (𝑘) is defined as: 𝑁𝑘 ∑ 𝑙 (𝑘) 𝐿̅𝑠 (𝑘) = 𝑛=1𝑁 𝑛 ⑼ 𝑘 where 𝑙𝑛 (𝑘) is the travel distance of vehicle n, And 𝑁𝑘 is number of vehicles awaiting at on-ramp s to merge into freeway. An indicator 𝛾𝑠 represents the preference in average travel distance along freeway is introduced at each on-ramp for higher-level FDC strategy. Eq. ⑺ and Eq. ⑻ can be transferred into: 𝑞𝑠 (k) = (1 − 𝛼𝑗 (𝑘)) (1 + 𝛾𝑠 + 𝛽) × 𝑑𝑠 (𝑘) ⑽ where 6 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) β= N−∑𝑠∈𝑆(𝑑𝑠 (k)×(1+𝛾𝑠 )) ∑𝑠∈𝑆 𝑑𝑠 (k) ⑾ 𝛾𝑠 is assumed to be no less than 0 and stands for the extent of preference at on-ramp s. After applying indicator 𝛾𝑠 all on-ramps target flow is enlarged by adding a non-negative 𝛾𝑠 in the multiplier bracket in Eq. ⑽ while the compensation parameter β is adjusted accordingly in Eq. ⑾ to meet the demand-capacity conservation. The key problem now becomes the determination of 𝛾𝑠 . The hypothesis above in 2.4.1 shows that shorter average travel distance is preferred than longer one, so that 𝛿𝑖 is also leaning towards to shorter average travel distance. The proposed way uses the inverse of average travel distance as weight factor which is formed in Eq. ⑿. 𝛾𝑠 = 𝜇 1 ̅̅̅̅ 𝐿𝑠 ⑿ 1 𝐿𝑠 ∑𝑠∈𝑆(̅̅̅̅) where 𝜇 is a sensitivity valve that balance the lower level ALINEA strategy and higher level HERO strategy. 𝜇 is 1 as default and need further calibration. In this situation, 𝛾𝑠 will be positive and fit in the range of (0, 1). 3 Case study 3.1 Model description A virtual network is used in this case study and the origin destination matrix is also artificial. The model is built in PARAMICS. The researched segment is 4.6 km long and contains 2 onramps with ramp meters and 2 off-ramps. Mainstream of this section is two-lane and all ramps are one-lane. The geometry brief diagram is shown in Figure 4. Figure 4 Freeway geometry There are three time periods in this simulation. The first period is a ten minute warm up period which has a very low demands (approximately 0). The second period is 60 minute long and the demand matrix for the second period is shown in Table 1. The third period is 50 minute long and the demand is zero since it is a clearing period that allows all vehicles to arrive their destination. In total, two hour is simulated in order to imitate peak hour traffic situation. Table 1 Demand matrix Origin\Destination Demand (veh/hr) A 𝑿𝟏 𝑿𝟐 B 349 170 3130 7 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) 𝑸𝟏 n/a 0 989 𝑸𝟐 n/a n/a 793 As we can see, the mainstream has a relatively high traffic flow while certain vacancy can be seen after off-ramps (𝑋1 and𝑋2 ). However the vacancy is not big enough to deal with the demands of on-ramps (𝑄1 and𝑄2 ). So certain congestion should occur on this section and ramp metering is applied to control on-ramps to avoid congestion. The HERO ramp metering mentioned in the last chapter is translated into plug-in which is readable for PARAMICS. The time step is set to 15 seconds, which means the traffic control centre collects data, processes data and applies ramp metering strategy every 15 seconds. The coordination distance in HERO is set to be 3 km. 3.2 Experiment design The simulation contain several scenarios, including: ALINEA only: ramp metering only depends on local detector stations (lower level only) ; and only applies ALINEA strategy. Average travel distance HERO: heuristic coordination strategy related to average travel distance along freeway (higher level) is applied to overlap ALINEA (lower level). No ramp metering: as a comparison, no ramp metering is performed in this case. 3.3 Results The results are observed from the detector stations set on the mainstream every 500 meters and the ramp metering log is generated by the plug-in when HERO makes decision. A comparison of ramp metering rate before and after coordination is represented in a ramp metering ratio log in Figure 5 and Figure 6. Figure 5 Ramp metering ratio at ALINEA only scenario ALINEA only scenario node 132 node 44 ramp metering ratio 1.2 1 0.8 0.6 0.4 0.2 0 0 50 100 150 200 250 300 350 400 450 500 time step ID In ALINEA only scenario, ramp metering ratio varies more rapidly and ramp metering ratio is more sensitive to local detector station data. However in average travel distance HERO scenario, ramp metering ratio is more smoothed due to the coordination operation between node 132 and node 44. As shown in Figure 5 and Figure 6, an observable coordination starts to kick in at about 150th time step (at 37.5 minute) and ends at about 300th time step (at 75 minute). Two phases can be observed, first phase when node 44 is preferred is from 150th time step to 219th time step, and the second phase when node 132 is preferred is from 220th time 8 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) step to 300th time step. Node 44 is preferred since its shorter travel distance in the first phase while node 132 gets more preference in the second phase because of its large demand. Figure 6 Ramp metering ratio at Average travel distance HERO scenario Average travel distance HERO scenario node 132 node 44 ramp metering ratio 1.2 1 0.8 0.6 0.4 0.2 0 0 50 100 150 200 250 300 350 400 450 500 time step ID To further prove what ramp metering ratio represents, cumulative flow diagrams of node 132 and node 44 are shown in Figure 7 and 9 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) Figure 8 respectively. Same flow pattern as the ramp metering ratio is shown. Figure 7 Cumulative flow diagram of node 132 ramp node 132 1600 1400 1200 1000 800 600 400 200 0 hero nothing alinea 10 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) Figure 8 Cumulative flow diagram of node 44 ramp node 44 1000 800 600 400 200 0 hero nothing alinea As you can see, the ramp metering regulated traffic counts are less than the case without ramp metering. This represents that ramp metering algorithms are functioning to prevent freeway to be merged with too many vehicles and then prevent freeway from congestion. To better understanding what are the two ramp metering algorithms, several loop detector data have been derived from the experiments to demonstrate the functions of HERO and ALINIEA comparing to the “no ramp metering” case. Figure 9 Cumulative number of vehicles at mainstream before ramp node 132 mainstream before ramp 132 4000 3500 3000 2500 2000 1500 1000 500 0 hero nothing alinea 11 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) Figure 10 Cumulative number of vehicles after ramp node 132 mainstream after ramp node 132 6000 5000 4000 3000 2000 1000 0 hero nothing alinea Figure 11 Cumulative number of vehicles before ramp node 44 mainstream before ramp node 44 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 hero nothing alinea 12 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) Figure 12 Cumulative number of vehicles after ramp node 44 mainstream after ramp node 44 6000 5000 4000 3000 2000 1000 0 hero nothing alinea From Figure 7 to Figure 12, we can observe that the mainstream cumulative number of vehicles are similar between the three experimental scenarios, which means the mainstream have been fully occupied after the first onramp merging(Figure 10, Figure 11 and Figure 12). The differences are the on-ramp merging number of vehicles and mainstream flowing vehicles. In Figure 7 and Figure 8, no ramp metering (“nothing”) case merged more vehicles into the freeway than ramp metering cases (“hero” and “alinea”) and in Figure 9, fewer mainstream flow are allowed to enter this section of freeway. This means that more vehicles are congested from the upstream. In addition, total delays on freeway links are calculated in all scenarios respectively. The total delay under no ramp metering scenario is 15622656.0 seconds. The total delay on freeway links in ALINEA only scenario is 354205.1 seconds while the total delay on freeway links in average travel distance HERO scenario is 223351.3 seconds. The delay results further prove that ramp metering algorithms (both HERO and ALINEA) prevent freeway from congestions and maintain freeway in an efficient condition during the peak hour. In the terms of performance, the developed HERO algorithm provides more travel time saving than ALINEA while maintaining freeway in similar uncongested traffic flow. 4 Conclusion In this paper, a new coordinated ramp metering algorithm is developed which not only utilizes latest VII system but also is capable of processing vehicle routes. A hypothesis is proposed that vehicles that travel shorter distance along freeway are preferred. A hypothetical freeway section model is built in PARAMICS to test the new algorithm. Results show that both HERO and ALINEA strategy are effective in reduce mains stream congestion and provide travel time savings than no ramp metering situation. In addition, the innovated coordinated ramp metering strategy (average travel distance HERO) outputs more smoothed ramp metering ratio than local ramp metering strategy (ALINEA). Benefit is gained in travel time saving in coordinated ramp metering strategy (HERO) comparing to local ramp metering strategy (ALINEA). 5 Future works A guess is that there is still room for new HERO strategy to gain more benefit by adjusting the balance valve μ. By tuning μ, maybe new HERO strategy would pick up the ramp metering ratio sensitivity in lower level ALINEA and the travel time saving in higher level HERO. So more work certainly need to be done in the future. 13 A prospective way of ramp metering using vehicle infrastructure integration system (Tuo Mao1, Vinayak Dixit2) More methodology needs to be worked out to process vehicle routes information to make better understanding of traffic demands. Not only heuristic ways but also traffic dynamic models need to be researched. 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