Ramp Metering Summary Report

Transcription

1 Ramp Metering

2 Contents Section Page 1. Introduction Ramp Metering Concept International Experience Background and History Deployment of 30 Ramp Metering Sites Structure of Document 5 2. Ramp Metering Overview Introduction Ramp Metering Operation Ramp Metering Limitations 6 3. System Operation System Architecture Release Algorithm Arbitration Algorithm Switch On-Off Algorithm Queue Override Algorithm Queue Management Algorithm Ramp Metering Algorithm Data Filtering Algorithms National Deployment of 30 Ramp Metering Sites Initial 30 Sites HA Areas 9 & HA Area HA Area Further Implementation Site Selection Design Installation System Commissioning Calibration 20 November 2007 i _04_02_141

3 5.6 Operation and Maintenance Evaluation Economic Benefits Ramp Metering Evaluation Evaluation Methodology Data Collection Evaluation Findings Further Ramp Metering Development Introduction Review of Ramp Metering Operation Enhancements to Aid Set Up, Calibration and Maintenance Enhancements to Improve Performance at Standard Locations Enhancement to Optimise Operation at New Types of Location Implementation of Enhancements Conclusions References Further Reading 9-28 List of Tables Table Area 9 &11 Sites 16 Table Area 10 Sites 17 Table Area 12 Sites 17 Table Limits of Variables to Consider when Assessing Site Suitability 19 List of Figures Figure 3.1: Negative Feedback Control Loops of Ramp Metering Process 8 Figure 3.2: System Overview of the Ramp Metering System 9 Figure 3.3: Release Algorithm Schematic 10 Figure 3.4: Arbitration Algorithm Schematic 10 Figure 3.5: Switch On-Off Algorithm Schematic 11 Figure 3.6: Queue Override Algorithm Schematic 12 Figure 3.7: Queue Management Algorithm Schematic 12 Figure 3.8: Proportional Occupancy Queue Management Algorithm Schematic 13 Figure 3.9: Ramp Metering Algorithm Schematic 13 Figure 3.10: ALINEA Algorithm Schematic 14 Figure 4.1: Current Ramp Metering Installation 15 Figure 9.1: Flowchart of deployment project life cycle documentation 9-27 November 2007 ii _04_02_141

4 1. Introduction 1.1 Ramp Metering Concept Ramp metering is a traffic management technique which regulates the manner in which vehicles join a motorway at peak periods. The purpose of the system is to prevent or delay the onset of flow breakdown on the main carriageway by a combination of: Regulating the flow of additional traffic onto the motorway that, if unregulated would trigger flow breakdown; and, Managing the flow on the entry slip road to avoid large platoons of vehicles entering the main carriageway and causing flow breakdown. By preventing or delaying flow breakdown the system provides the following benefits: Greater throughput during peak periods; Less congestion and improved traffic flows; Smoother and more reliable journey times; Reduced risk of accidents; and, Environmental improvements as a result of noise reduction and improved fuel consumption. The ramp metering system uses part time signals on the slip road which come into operation when traffic sensors on the main carriageway indicate heavy traffic. The full operation is described in MCH1965 Ramp Metering System Requirements Specification. 1.2 International Experience Ramp metering is not a new concept. In the USA it started in the early 1960s and there are now over 2,200 ramp metering sites there. Some European countries have operated systems since the 1980s, with ramp metering being implemented in Germany, the Netherlands, France and Belgium. In 2001, the Euro-Regional project, CENTRICO, compiled a review of ramp metering in these countries. Other countries have also conducted or are planning trials. In general, ramp metering sites operate in three different methods: 1. Time of day (fixed rate). 2. Incident / special event (manually switched). 3. Traffic responsive (continuously operating). They are configured as either stand-alone or centralised control systems (the latter configuration was implemented in Seattle in the early 1990s to allow a more strategic approach to congestion management by providing feedback from downstream ramp metering sites and local roads). November _04_02_141

5 The following ramp metering strategies have been used in various countries: Fixed rate with look up tables; Demand-capacity; ALINEA (widely used in Europe); and, Adaptive ramp metering strategies (e.g. California). Operationally they vary with the following main differences: One car per green (usually with appropriate signing); Not all use an amber signal; Belgium and Minneapolis use flashing ambers when ramp metering is not operational; In Bordeaux, a flashing amber signal is used to indicate that traffic can proceed; In Munich, the system used a non illuminated open aspect to indicate traffic can proceed; and, Cycle times vary between 4 seconds and 40 seconds. One European report, summarising the experience of ramp metering at 8 sites in the Netherlands over the past ten years, admits that it is difficult to make a judgement about the effects of ramp-metering. An overview of the systems and ramp metering experience in the US and Europe indicates that the benefits achieved can vary and also depend on which site is considered. However, generally the claimed benefits can be summarised as: Throughputs were increased; Mainline speed increased; Travel times were reduced on the main line; Emissions were reduced (one US scheme in Denver - evaluated this); and, Accidents were reduced. Any delays to slip road traffic must be offset against these benefits. 1.3 Background and History The first ramp metering system in England (installed on the M6 J10 in May 1986) was designed for the West Midlands Regional Office of the Department of Transport by Wootton Jeffreys and installed by Plessey Controls. The Ramp Metering Pilot Scheme (RMPS) (installed on both M6 and M27 motorways) was originally commissioned by The Highways Agency (HA) in 1998 with Owen Williams as managing consultants, supported by IPL for the NMCS Design. Transport Research Laboratory (TRL) was responsible for site selection and scheme assessment, having previously been commissioned by the Highways Agency in 1997 to identify suitable sites on the M3/ M27. November _04_02_141

6 The ramp metering pilot project was completed in 2005 with the final evaluation undertaken by a Highways Agency team of Atkins, MMB Associates, Technical University of Crete and University of Southampton. Following the successful completion of the pilot project an initial deployment of 30 ramp metering sites on the Highways Agency network was undertaken. 1.4 Deployment of 30 Ramp Metering Sites In 2005, the Highways Agency (HA) undertook a project to deliver 30 ramp metering systems by the end of March These ramp metering systems are located in the West Midlands and the North of England. They have been installed in the following Highways HA MAC Areas: Area 9 10 sites; Area sites; Area 11 1 site; and, Area 12 7 sites. 1.5 Structure of Document The document comprises the following sections: Section 1 basic introduction to the document; Section 2 ramp metering theory; Section 3 ramp metering system operation; Section 4 the national deployment of 30 sites; Section 5 advice for further implementation; Section 6 details the effectiveness of ramp metering; Section 7 planned further development; Section 8 conclusions of the various ramp metering projects; and Section 9 related documentation and further reading. November _04_02_141

7 2. Ramp Metering Overview 2.1 Introduction Ramp metering is a tool for managing motorway traffic by controlling the flow of traffic on entry slip roads. The aim of ramp metering is to regulate flow from the slip road on to the main carriageway at a level that maximises throughput on the main carriageway. 2.2 Ramp Metering Operation When busy traffic conditions exist on the main carriageway, the introduction of traffic from the slip road can cause vehicles to change lanes or brake which gives rise to higher occupancy and lower headways. These shorter headways can be unsustainable at the speed of the main carriageway. For comfort and safety, drivers will adjust their speed to account for the short stopping distances available. This adjustment of headway can occur over a distance of up to 2km after the on slip. Often this adjustment of headway will cause following vehicles to brake, propagating a wave of braking vehicles in the traffic stream. Traffic concentration in the wave will be even higher. To compound the problem, more vehicles will be entering the main carriageway boosting concentration even higher. If vehicles continue to join, ultimately the main carriageway speed will drop to a point where flow breakdown occurs. In this situation vehicles are stopping at the back of a queue and then driving off the front of the queue. This stationary traffic is typically seen between the merge area and approximately 2km downstream. As a result of standing traffic, sometimes called a phantom jam, the road effectively has its lowest throughput when the demand is at its highest. Weather conditions, daylight, vehicle mix and gradients amongst other things can all affect the maximum throughput of any section of motorway. To address this problem, ramp metering aims to maximise throughput on the main carriageway without disrupting the local road network. It does this by controlling the discharge of traffic from the slip road to reduce the interference of merging traffic on the main line flow thereby maintaining flows at a higher level. Maintaining higher flows will postpone the onset and reduce the duration of flow breakdown on the main carriageway. The ramp metering system relies on the measurement of traffic conditions on the main carriageway and attempts to maintain this at target occupancy by restricting the flow from the on slip road. 2.3 Ramp Metering Limitations There are some congestion problems where ramp metering systems may not provide effective benefits. These are congestion problems caused by bottlenecks downstream of a slip road or where the flow from the slip road is too low. In this situation the slip road flow, whether restricted or not, is unlikely to have a significant impact on the traffic flowing through the bottleneck. November _04_02_141

8 Situations such as this are particularly relevant when: Flow from the slip road is low compared to main carriageway flow. The bottleneck problem causes a large congestion problem, where capacity of the road is greatly exceeded. Large bottleneck problems would typically include a large change in capacity on a road for example: - Capacity of road reduction due to lane loss; - Traffic backing up from an off slip and blocking a lane of the main carriageway; - Diverging tailbacks at motorway intersections; and, - Roadwork Traffic Management/ Accident causing lane loss. November _04_02_141

9 3. System Operation 3.1 System Architecture The ramp metering process uses negative feedback control loops as illustrated in Figure 3.1 below. The negative feedback control loop algorithms strive to maintain maximum throughput on the main carriageway without disrupting the local network roads. Processing On/Off, Queue Management, Queue Override, Ramp Metering. Outputs Arbitration & Release Algorithms / Signal Times Inputs Filtered / Smoothed Traffic Data Motorway Figure 3.1: Negative Feedback Control Loops of Ramp Metering Process There are seven algorithms used by the current ramp metering operation as follows: Release algorithm; Arbitration algorithm; Switch on-off algorithm; Ramp metering algorithm; Queue override algorithm; Queue management algorithm; and, Data filtering algorithm(s). Generally, the traffic data is monitored by the main carriageway data filtering and ramp data filtering algorithms. The traffic data is used in the ramp metering control algorithms to determine the best traffic flow for the system to operate efficiently. This traffic flow is converted into a period of signal sequences to the motorists, in order to dissipate the intended traffic flows onto the main carriageway. The principal control algorithms used are shown in Figure 3.2 as follows: Switch on-off algorithm - turns the system on and off; November _04_02_141

10 Queue override algorithm prevents the overload of slip road queue to avoid congestion in the local road network; Queue management algorithm using proportional occupancy manages the queue on the slip road; Main carriageway algorithm using ALINEA or demand capacity controls the main carriageway occupancy / flow. Monitor Switch Algorithms Release On/Off Main Carriage- Way Desired Flow Ramp Desired Flow Main Carriageway Queue Management Queue Override Arbitration Release Traffic Behaviour Figure 3.2: System Overview of the Ramp Metering System 3.2 Release Algorithm The release algorithm sets the traffic signals appropriately to provide the required traffic flow released from the slip road. As a secondary function, the release algorithm also monitors the actual release rate. There are 10 different sets of signal times, each provides a distinct level and pattern of traffic flow from the stop line onto the main carriageway. The release algorithm also manages the transition from signals off to signals on, and vice versa, via a steady green state of a pre-defined minimum duration. Figure 3.3 below shows the release algorithm schematic. November _04_02_141

11 Signal & Loop Faults / Queue Presence & Actual (Release) Flow (r act ) Required Slip Road (Release) Flow (r) Signal Timings & Sequencing Presence & Release Information Figure 3.3: Release Algorithm Schematic 3.3 Arbitration Algorithm The arbitration algorithm manages the required slip road flow from the ramp metering, queue management, queue override and switch on-off algorithms and determines the correct slip road release flow to pass on to the release algorithm. The outputs of the ramp metering, queue management, queue override and switch on-off algorithms provide the input to the Arbitration algorithm. Each of these outputs is re-calculated at different intervals. As such, the arbitration algorithm constantly monitors all of the desired flow rates and re-calculates the correct release flow output to the release algorithm in real time. Figure 3.4 below shows the arbitration algorithm schematic. The arbitration algorithm always selects the highest desired release flow from the input algorithms and passes that value to the release algorithm. Ramp Metering Release Flow (r rm ) Queue Management Release Flow (r qm ) Required Release Flow (r) Queue Override Release Flow (r qo ) Switch On-Off Release Flow (r oo ) (max) Arbitration Algorithm Release Algorithm Figure 3.4: Arbitration Algorithm Schematic November _04_02_141

12 3.4 Switch On-Off Algorithm The switch on-off algorithm switches the ramp metering system on or off. It switches off the ramp metering system by setting its desired release level to the pre-defined highest value flow rate. Likewise, it switches on the system by reducing its desired flow rate until the desired flow rate drops below the outputs from the other algorithms. It switches on or off by monitoring the traffic conditions, when minimum operational levels of occupancy or flow and occupancy are exceeded. It also initiates the switch off sequence or delays switch on if the main carriageway speed is at or above the pre-determined safe operational speed. In order to avoid releasing a full slip road of queuing vehicles into the mainline, the switch on-off algorithm monitors the occupancy of the two queue presence loops and holds the switch off sequence at the maximum release rate until the queue at the stop line has dissipated. Figure 3.5 below shows the switch on-off algorithm schematic. There are five operational modes in the switch on-off algorithm, namely: Manual On operational when the upstream mainline speed is low enough; Manual Off overwrite mode to turn the system off manually; Timed operational when the day and time and maximum speed criteria are met; Timed Occupancy operational when the day and time of day, minimum occupancy and maximum speed are met; and, Timed Flow and Occupancy operational when the day, time, minimum flow and minimum occupancy are met. Main Carriageway Occupancy (O out ) Main Carriageway Flow (Q out ) Time of Day (T) / Manual Override f oo (k oo ) Desired On-Off Release Flow (r oo ) Main Carriageway Speed (V in) Queue Presence (O qp ) Switch On-Off Algorithm Arbitration Algorithm Figure 3.5: Switch On-Off Algorithm Schematic November _04_02_141

13 3.5 Queue Override Algorithm The queue override algorithm reduces the queue of traffic waiting to join the main carriageway to prevent the queue from adversely affecting the local roads. It detects the presence of an excessive queue length and reduces the queue length immediately by releasing the traffic from the slip road to generate a sufficient space within the slip road to prevent disruption to the local road network. Operation of the queue override algorithm is at the expense of the main carriageway. Figure 3.6 below shows the queue override algorithm schematic. Occupancy at Maximum Extent of Queue on Slip Road (O qo1 to O qo2 ) f qo (k qo ) Desired Queue Override Release Flow (r qo ) Queue Override Algorithm Arbitration Algorithm Figure 3.6: Queue Override Algorithm Schematic 3.6 Queue Management Algorithm The queue management algorithm controls the queue length to acceptable limits to maximise the period of effective ramp metering operation whilst minimising the operation of the queue override function. Figure 3.7 below shows the queue management algorithm schematic. Desired Ramp Release from Proportional Occupancy Queue Management Algorithm (r poqm ) Desired Ramp Release from Weighted Occupancy Queue Management Algorithm (r woqm ) Queue Management switch Desired Queue Management release flow (r qm ) Arbitration Algorithm Figure 3.7: Queue Management Algorithm Schematic The proportional occupancy queue management algorithm monitors the occupancy at each set of queue detection loops to obtain the average occupancy of the slip November _04_02_141

14 road, which is used to estimate the queue length. Based on the estimated queue length, the proportional occupancy queue management algorithm sets the desired release rate to maintain the queue length at the pre-defined desired value. Figure 3.8 below shows the proportional occupancy queue management algorithm. Desired Queue Occupancy (O descq ) Measured Combined Queue Occupancy (O cq ) (r rm ) Calculated Ramp Flow (r poqm ) Figure 3.8: Proportional Occupancy Queue Management Algorithm Schematic 3.7 Ramp Metering Algorithm The ramp metering algorithm determines the optimum traffic flow from the slip road to control the flow of traffic on to the main carriageway. Figure 3.9 below shows the ramp metering algorithm schematic. Desired Ramp Release from Demand Capacity Algorithm (r dc ) Desired Ramp Release from ALINEA Algorithm (r al ) Ramp Metering Switch Desired Ramp Metering Release Flow (r rm ) Arbitration Algorithm Figure 3.9: Ramp Metering Algorithm Schematic The ALINEA algorithm outputs the required slip road flow value in proportion to the difference between the measured downstream occupancy and the desired downstream occupancy. If the downstream occupancy is greater than the desired November _04_02_141

15 value, ALINEA reduces the slip road flow. However, if the downstream occupancy is less than the desired value, it increases the slip road flow. Hence, the subsequent measured downstream occupancy is maintained as close as possible to the desired value. Figure 3.10 below shows the ALINEA algorithm schematic. Downstream Desired Occupancy (O des ) Calculated RampF (r dc ) Measured Downstream Occupancy (O out ) Figure 3.10: ALINEA Algorithm Schematic 3.8 Data Filtering Algorithms Data filtering algorithms calculate smoothed values for flow, speed and occupancy from the raw vehicle data collected by the detectors. The smoothed values are used by the other algorithms outlined in this section. There are two data filters as follows: 1. Main carriageway data filtering algorithm. 2. The main carriageway data filtering algorithm calculates the smoothed flow, speed and occupancy data from the raw vehicle from the main carriageway traffic detectors. It calculates the smoothed flow, speed and occupancy for every loop pair at every loop site. Ramp data filtering algorithm. The ramp data filtering algorithm calculates the smoothed flow and occupancy values for the slip road activity. The RDF algorithm also calculates the number of vehicles passing each slip road loop to be used in the queue estimation and control algorithms. November _04_02_141

16 4. National Deployment of 30 Ramp Metering Sites 4.1 Initial 30 Sites This is the first widespread roll out of ramp metering on the Highways Agency network following the successful completion of the Ramp Metering Pilot Project. In 2005, the HA undertook a project to deliver 30 ramp metering systems by the end of March The ramp metering systems are located in the West Midlands and the North of England. They have been installed in the following Highways HA MAC Areas: Area 9: 10 sites Area 10: 12 sites Area 11: 1 site Area 12: 7 sites The scheme was aimed at standardising and addressing most of the issues raised during previous ramp metering projects. Developments were made in addressing the issue of slip road traffic overflowing. The ramp signals were post-mounted downstream of the stop line on either sides of the slip road. Slip road message signs warned drivers of the ramp signals in operation. The operation was automatic. An overview of the general installation in the ramp metering project is shown in Figure 4.1 below: Figure 4.1: Current Ramp Metering Installation November _04_02_141

17 In addition a pilot project has been undertaken on the M1 in South Yorkshire linking the ramp metering to the local roundabouts using Integrated Traffic Management. 4.2 HA Areas 9 &11 There are ten sites in total in Area 9, located on the M5, M6 and M42 around Birmingham. A further single site is located in HA Area 11 at M6 J11 southbound near Crewe. The table below lists all the sites within HA area 9 &11. Sites M6 J16 Southbound (New) M6 J11 Southbound (New) M6 J10 Northbound (Upgrade) M6 J10 Southbound (Upgrade) M6 J9 Northbound (Upgrade) M6 J9 Southbound (Upgrade) M6 J7 Southbound (Upgrade) M6 J5 Southbound (Upgrade) M5 J1 Northbound (New) M5 J1 Northbound (New) M42 J3 Eastbound (New) Status Table Area 9 &11 Sites 4.3 HA Area 10 The Area 10 sites were the first 10 sites to be installed and calibrated, with all sites (excluding M6 J23 NB) operational and calibrated by the end of July The table below lists all the sites within HA area 10. Sites M60 J2 Clockwise M62 J19 Eastbound M62 J11 Eastbound M6 J25 Southbound Status November _04_02_141

18 Sites M6 J24 Northbound M6 J23 Northbound M6 J23 Southbound M6 J22 Northbound M6 J22 Southbound M6 J18 Northbound M56 J2 Eastbound M60 J16 Eastbound Status Table Area 10 Sites 4.4 HA Area 12 There are seven sites situated in HA Area 12, six sites are located on the M1 and one site on the M62. The table below lists all the sites within HA area 12. Sites M62 J25 Eastbound M1 J41 Northbound M1 J40 Northbound M1 J39 Northbound M1 J35A Southbound M1 J35 Southbound M1 J33 Southbound (Anti- Clockwise) Status Table Area 12 Sites November _04_02_141

19 5. Further Implementation HA Traffic Operations Directorate are currently implementing further sites throughout the HA s Motorway network. This further work is to be performed in 3 phases as follows: Phase 1: 25 sites to be installed by March 08; Phase 2: up to 45 more sites to be installed by March 09; and, Phase 3: ongoing development of the system operation to provide further benefits. The implementation of a typical ramp metering scheme can be split into stages as detailed in Figure 9.1. Each stage can be defined individually but aspects from each will overlap and impact on the next. The sections below give a brief description of the different stages of ramp metering scheme life cycle. A more detailed description can be found in the documentation referred to in Figure Site Selection This is primarily based on the traffic flows in and around each junction, with considerations being made to junction layout and topography. Detailed guidance has been prepared to help route managers assess ramp metering implementation. These criteria have been derived from experience gained with existing ramp metering schemes on the motorway network. The two main factors affecting the feasibility of a site for ramp metering are: Traffic characteristics; and, Physical characteristics. A summary of the criteria for ramp metering installation is given in Table 5.1. The table is updated from the experiences gained from the national roll out of ramp metering. November _04_02_141

20 Parameter Minimum Value Maximum Value Average mainline speeds in the vicinity of the merge during congestion (kph) Annual Delay at speeds below 50kph. Downstream mainline flows per lane (vph) Slip road flows per lane (vph) Slip road flow as percentage of downstream flow (%) Ideal Acceptable Ideal Acceptable No Minimum Value 70 10,000 Vehicle Hours Delay 1, Hours average speed below 50kph Appreciable based on local knowledge No Maximum Value No Maximum Value , Table Limits of Variables to Consider when Assessing Site Suitability 5.2 Design Full details of system design considerations are available from the Highways Agency. The underlying principle behind any design is the operational strategy. The designer has responsibility for developing the design intent to ensure that the system components can be built and operated in a safe, efficient and cost effective manner as required by the operational strategy. It is essential to the success of a ramp metering project that the calibration team take due cognisance of the operational strategy and design intent. 5.3 Installation The installation of the infrastructure is carried out by a designated contractor. This element includes installing all signs and signal poles, anti-skid surfacing, cabinet sites, ducting and cabling and slip road loops. 5.4 System Commissioning The commissioning phase is completed once all infrastructure elements are completed. A default configuration file is loaded on to the system and a series of tests are undertaken to ensure that each system (hardware and software) has the intended functionality. November _04_02_141

21 5.5 Calibration During this phase the system is finely tuned to optimise performance. This process is a careful balance between the slip road and main carriageway activity. From the operational strategy generated during the design phase, values for the numerous parameters are derived to give an initial setup. This initial setup is then finely tuned whilst the system is live and metering traffic. At the end of this phase the system will be fully operational. Unsuitable calibration parameters can easily cause additional and unnecessary delays to the mainline and / or the slip road and local road network. 5.6 Operation and Maintenance As the ramp metering system is designed as a stand alone system the operational impacts are negligible. The ramp metering system will adapt to normal changes in traffic flows during summer and winter and will operate automatically only when the mainline conditions require. 5.7 Evaluation The detailed setup at each site should be evaluated to ensure it is giving the optimum performance. Continuous evaluation and re-calibration may be required if the traffic flows around the area of a particular junction change. This could be due to a HA scheme in the vicinity of the junction or increased traffic on the slip road due to changes in the local area or road network. Economic benefit calculations for a proposed ramp metering scheme should be based on delay reductions of between five and ten percent depending on how well the site traffic conditions meet the requirements for ramp metering installation. 5.8 Economic Benefits The economic case for ramp metering was based upon an assumed 5% reduction in vehicle hours delay on the mainline carriageway during peak operational periods. This reduction in delay equates to an approximate five second saving per vehicle on the mainline over a typical two hour peak period. This was considered to be a conservative estimate of the overall benefits of ramp metering and resulted in an average first year rate of return of 7%. November _04_02_141

22 6. Ramp Metering Evaluation 6.1 Evaluation Methodology The evaluation methodology employed to assess the impact of the first 30 ramp metering sites uses the following main indicators to measure the operational performance of the sites with ramp metering before and after installation: Journey times; Traffic speeds; and Traffic flows. The focus of the operational evaluation method is based on calculations of journey times, journey speeds, and traffic flow before and after installation of ramp metering. The main source of collecting data for evaluation is through MIDAS loops, typically spaced at 500m intervals along the mainline carriageway (MIDAS stands for Motorway Incident Detection and Automated Signalling; more information regarding this can be found on the Highways Agency website). 6.2 Data Collection The evaluation period for each ramp metering site was based on a 20 day sample of both OFF and ON data free of untypical events and weather. Typically, OFF data was collected prior to the installation and calibration of the ramp metering system, with the ON data collected once the site was fully calibrated and operational. The 20 days of data were then averaged to produce an average journey time, traffic flow and speed over the evaluation period. The data collected has been extensively checked and filtered to ensure that as far as reasonably possible, the evaluation is based upon a true and fair comparison of traffic conditions in the OFF and ON data sets. The process that has been used to obtain the evaluation data is as follows: Acquire MIDAS Traffic Data; Examine the data to identify and exclude abnormalities such as missing data; Identify and exclude days with incidents using visual inspection of Motorway Traffic Viewer (MTV) plots and Regional Control Centre (RCC) Incident data logs; Classify and exclude days according to rainfall data obtained from HA weather monitoring sites in each area; Identify the typical operational periods for each site from the RM system log files; Calculate journey times using MIDAS data and MTV software; and, Analyse MIDAS traffic data for ON and OFF periods in order to derive spot speeds, traffic flow throughput, speed flow-curves at key upstream and downstream locations for each site. November _04_02_141

23 6.3 Evaluation Findings The overall increase in peak period traffic flows observed on the mainline after the installation of ramp metering varies by site with individual increases in traffic flow ranging from 1 8%. Despite the increases in traffic flow the implementation of ramp metering has resulted in downstream traffic speed increasing by between 3.5% and 35%. In many cases, ramp metering operation has led to a delay in the onset of flow breakdown and/ or earlier recovery from flow breakdown conditions. The large variation in the encountered benefits can be explained by the range of different road and traffic conditions existing at the junctions (e.g. downstream traffic disruptions, weaving for approaching downstream junction etc.). A full evaluation is available from the Highways Agency describing the benefits and conditions encountered in detail. Overall journey time savings on the mainline during peak periods of up to 40% have been reported, with an average journey time saving for mainline traffic of 13% across all sites evaluated. The average on-slip delay per vehicle with ramp metering operational ranged from 15s to 78s, however the sites with the highest delay on the slip road in general also delivered the highest benefit on the mainline. The initial economic assessment of ramp metering was undertaken on the basis of an assumed 5% reduction in delay for mainline traffic, which resulted in a 7% First Year Rate of Return (FYRR) averaged across all thirty sites. The revised economic assessment, which has been based upon the observed mainline journey time savings, together with the calculated slip road delays, indicates that ramp metering yields significantly higher returns than those assumed in the initial evaluation, with FYRR ranging from 7% through to 98%, with an average FYRR for those sites evaluated of 48%. The observed FYRRs are much higher than those derived in the initial economic assessment due to the fact that the actual journey time savings, as a result of implementing ramp metering, are much higher than those assumed in the initial appraisal. The operational assessment of ramp metering has indicated that the system has resulted in significant benefits to the travelling public on the mainline of the motorway in terms of journey time savings, increased speeds, increased traffic flows and more stable conditions. In addition the system results in economic benefits with significant returns in its first year of operation. Operationally, ramp metering has proved to be a great success. November _04_02_141

24 7. Further Ramp Metering Development 7.1 Introduction The 1980s deployment of ramp metering utilised the most advanced control technology of the time, namely an Urban Traffic Control System (UTC). There were however, significant limitations on communications and processing power with this system. Advances in communications and control technology over the last 20 years and advances in the understanding of traffic behaviour have led to significant refinements in ramp metering technology within the UK and internationally. The ramp metering pilot project utilised National Motorway Control Systems 2 (NMCS2) technology based around the MIDAS sub system and MIDAS outstation. The evaluation of the pilot project led to recommendations being made to optimise the technology for a wider deployment. The enhanced technology was used for the initial deployment on 30 sites. The experience of the deployment of 30 sites resulted in a review of the various aspects of the design, configuration and optimisation of ramp metering. The outcome of the review and recommendations for continuous deployment are summarised in this section. 7.2 Review of Ramp Metering Operation The review of the operation of ramp metering focused on three main areas, which were as follows: Identification of changes to reduce the set up, calibration and maintenance activities; Identification of changes to improve performance at standard locations; Identification of changes to optimise the operation new types of location including Roadworks, Controlled Motorway, Active Traffic Management (ATM) and motorway to motorway locations. Once the review was completed recommendations were made for a phased approach to implementation of the identified changes. Details of the proposed changes and the recommendations can be found in Ramp Metering Algorithm Refinements - Final Report (reference _06_05)._. 7.3 Enhancements to Aid Set-up, Calibration and Maintenance The following enhancements to aid the set up, calibration and enhancement of the system have been identified: Improved data logging; Addition of Adaptive ALINEA algorithm for main carriageway control; November _04_02_141

25 Automatic calibration of signal timings; Accurate calculation of vehicles released per cycle; Automatic calibration of some Queue Management (QM) and Queue Override (QO) parameters; and, Implementation of rules of thumb in software. 7.4 Enhancements to Improve Performance at Standard Locations The following enhancements to improve performance have been identified: Linked sites algorithm; Merge control algorithm; ALINEA cascaded with Demand Capacity (DC) method known as ACDC; Junction Active Management (JAM); and, Dual stop lines. 7.5 Enhancement to Optimise Operation at New Types of Location The following enhancements were identified to address issues relating to new types of location including road works, Controlled Motorway, ATM and motorway to motorway locations. The proposed enhancements included: Algorithms relating to link with Controlled Motorways and ATM systems; Motorway-to-motorway queue-balancing algorithm; Use of alternative sensors; Disabling of detection during lane closures; and, Automatic detection of lane closures and contra-flow. 7.6 Implementation of Enhancements The recommendations for implementation categorised the changes into short, medium and long term. The recommendations for short term changes were as follows: Improved data logging; and, Automatic detection of lane closures and contra-flow. The recommendations for medium term changes were as follows: Addition of Adaptive ALINEA algorithm for main carriageway control; Automatic calibration of signal timings; Accurate calculation of vehicles released per cycle; Automatic calibration of some QM and QO parameters; Implementation in software of rules of thumb; Linked sites algorithm; November _04_02_141

26 Merge control algorithm; and, Disabling of detection during lane closures. The recommendations for long term changes were as follows: Automatic data collection & analysis in design stage; Automatic data collection & analysis for evaluation; Motorway-to-motorway queue-balancing algorithm; ALINEA cascaded with DC (ACDC); Junction Active Management (JAM); Algorithms relating to link with Controlled Motorways and ATM systems; Dual stop lines; and, Use of alternative sensors. November _04_02_141

27 8. Conclusions The ramp metering pilot was successfully completed in Following the success of the pilot project, ramp metering has been successfully deployed at 30 sites on the Highways Agency Network. It is envisaged that ramp metering will be deployed more widely in the coming period. Guidance information on site selection has been produced. A plan for continuous development of ramp metering has been produced and is currently being implemented (Ramp Metering Implementation Project). November _04_02_141

28 9. References The following diagram shows the current Highways Agency documentation detailing the various stages of ramp metering scheme as detailed in section 5. Figure 9.1: Flowchart of deployment project life cycle documentation November _04_02_141

29 9.1 Further Reading European Ramp Metering Project, EURAMP website at Higginson R, McCabe K, Rayman N. Highways Agency Ramp Metering Pilot Project - Smart Moves Birmingham 2006 Hayden J, Higginson R, Kosmatopoulos E, McCabe K, Papageorgiou M, Rayman N. Real time estimation of the critical occupancy for maximum motorway throughput. In Transport Research Record: Journal of Transportation Research Board No 1959 TRB, National Research Council Washington DC 2006 pp Hayden J, Higginson R, Kotsialos A, McCabe K, Papageorgiou M, Rayman N Discrete release rate impact on ramp metering performance IET ITS Journal Papageorgiou M, Hadj-Salem H and Middelham F. ALINEA Local Ramp Metering Strategy Summary of Field Results Paper No Transportation Research Record Gould C, Hermans F, MacDonald M, McCabe K, Papageorgiou M, Rayman N, Schofield M, Sultan B. Ramp metering in a UK Context RTIC GouldC, Munro P and Hardman E (2002), M3/M27 ramp Metering pilot Schemeimplementation And Assessment, RTIC March Taale H and Middelham F (2000) Experiences in the Netherlands Paper for the 10 th International Conference on Road Transport Information and Control, IEE, London, April 2000 Conference Publication No. 472 IEE Gould C, Mackinnnon M,Munro P and Chappell P (2000), M3/M27 Ramp Metering Pilot Scheme- Inception and design, RTIC March Owens D and Schofield M J(1990), Motorway access control: implementation and assessment of Britain s first ramp metering scheme, Transport Research Laboratory Project Report 252, TRL Crowthorne,1990. November _04_02_141

30 Ramp Metering Got a question or comment? ha_info@highways.gsi.gov.uk 24 hours a day, 365 days a year Live traffic information hours a day, 365 days a week *Calls from BT landlines to 0845 numbers will cost no more than 4p a minute and to 0870 numbers no more than 8p per minute, mobile calls usually cost more. * * Publications Code: PR262/07 Highways Agency Publications, Bristol Crown copyright For wider motoring advice visit DirectGov