Raise the awareness of the fundamental traffic processes ... Motorway traffic conditions may be studied at a range ..... EPSRC in the U.K. under contract number.
Barriers to Motorway Traffic Operations, and their Potential Solution M. Brackstone, M. McDonald Transportation Research Group, Dept. of Civil and Environmental Engineering, University of Southampton, Highfield, Southampton, U.K., SO17 1BJ Abstract 1. Introduction This paper reports on the progress of the ‘Platform Grant in Motorway Operations’ project in the U.K, which addresses problems in the use of in-vehicle systems to aid motorway congestion and safety. The project is undertaking a number of scoping studies in order to: i)
Develop and extend understandings of the fundamental processes governing traffic flow on motorways, through measurement and modelling,
ii)
Apply the understandings to assess strategies to alleviate identified problems in a range of scenarios, through the use of ITS technologies, such as ACC (Adaptive Cruise Control) and Convoy driving,
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Raise the awareness of the fundamental traffic processes, behaviour and opportunities for change with highway authorities, network operators and manufacturers.
The five scoping studies will provide an in-depth understanding of motorway operations and include Driver Reactions to Congested Condition, Behaviour in an Emergency, Junction Operations, 'Next Generation' Advanced Vehicle Control and Safety Systems (AVCSS) and Implications to Flow Modelling and Theory. A review of the technical workplan for each of these topics is given in the paper, along with early results from the first study. Plans for using these results to establish paths to deployment for AVCSS are also introduced.
Congestion on motorways has been increasing for many years and has substantially reduced the effectiveness of what was designed to be a high speed efficient inter-urban transport system. Also, although accidents per vehicle kilometre are very low, motorways are often perceived by the public as dangerous due to a small number of severe, high profile, accidents. The use of modern technology to provide new opportunities to reduce congestion and increase safety is hampered by the lack of an adequately detailed understanding of the 'problems', and their causative mechanisms. The ‘Platform Grant on Motorway Operations’ is funded by the Engineering and Sciences Research Council of the U.K. government as a basis for fundamental studies to tackle these issues. Motorway traffic conditions may be studied at a range of levels of detail from macroscopic to microscopic. Macroscopic, measures the overall flow, and speed and density may be related empirically or through a variety of models. Microscopic investigations consider the behaviour of individual drivers in detail. This latter approach is by far the most powerful. Such microscopic models must take account of the large degree of variation and choice associated with human decision making, and the accuracy of the modelling of such processes must be confirmed for validity. The development of databases to support such models is difficult, however research in this area, including obtaining an understanding of both normative and 'emergency reaction' in drivers may be seen as the foundation on which reliable microscopic traffic models may be based.
Research to determine microscopic understanding is taking place in a number of countries. For example in Japan, high profile activities regarding in-vehicle ITS are being conducted by all manufacturers, particularly Toyota and Nissan [1]. In the U.S.A., NHTSA have been active in promoting the evolution of the 'science of AVCSS (Advanced Vehicle Control and Safety Systems)' along with the help of Ford and GM, with a particular emphasis on safety [2], that has recently resulted in the inception of the Intelligent Vehicles Initiative. In Europe, USAP and Renault in France are active in promoting general research concerning vehicle based systems [3], while in Germany a wide range of work is being undertaken by various institutions as part of the MOTIVE workplan, with Daimler Benz and BMW now expanding their concerns from purely in-vehicle systems, to the construction of infrastructure based congestion forecasting and driver advice systems [4]. In the U.K. several high level groups exist for the planning of future activities (eg. PROMOTE, Foresight Vehicle, RTA) although the level of activity is less than for those mentioned above.
2.1 Driver Reaction to Congested Conditions. In general, existing microscopic simulation models have been calibrated using data which does not include flow breakdown. This is primarily due to a lack of adequate microscopic behavioural data on car following and lane changing in such circumstances. The goal of this phase has been to use the TRG instrumented vehicle (Fig 1) to measure these processes. In particular driver glance location, duration, and pedal movement have been measured in order to formulate a model of how drivers manage their distance keeping behaviour in 'critical' scenarios. Although an initial understanding of car following has been attained through earlier work [5], several important factors remain unexplored and this phase has attempted to address these, rather than attempt to attain a 'large scale' normative database.
The programme of research in this project will enable clear guidance to be developed on the efficiency, capacity and safety impact of many of these new technologies. It will support increased foresight in the planning of paths to deployment, future targeted research initiatives, and operational guidance/ standards.
2. Technical Work The project is investigating five main topics: • Driver Reaction to Congested Condition. • Driver Behaviour in an Emergency. • Fuzzy Modelling of Motorway Junctions. • Design/Test of Next Generation Advanced Vehicle Control and Safety Systems. • Implications to Flow Modelling and Theory. Each of these are introduced in the following sections.
Figure 1. The TRG Instrumented vehicle used for data collection. Data collection has concentrated on addressing four questions (more details on which may be found in [6]): 1. How does following behaviour vary with local conditions such as traffic flow density? 2. In these conditions, is there evidence of driver behaviour being affected by the motion of motorists several vehicles ahead? 3. How do motorists use the pedals to control the motion of the vehicle? Is it possible to quantify the situations in which drivers decelerate only by 'easing off' as opposed to activating the brake?
To what degree do drivers spread their attention between differing tasks, and how does this effect headway maintenance.
Data has been collected from local motorway sections (predominantly on the M27 and M3) during evening peak periods, and trials undertaken using a mix of 'passive mode' (radar fitted rearward observing following drivers) for question 2, and 'active mode' (facing forward assessing the actions of a test subject) for questions 3 and 4, with both being used for question 1. Analysis of the data is being undertaken using three approaches: Deterministic and probabilistic relationships between speed and desired following distance (Fig 2): Variations in reaction times and driver pedal use: Variation of the following distance and accelerative response with headway and relative speed ('spiral functions' see Fig. 3), from which thresholds for perceiving changes in relative speed and distance can be identified. Measurements are also being made using both collection modes for driver lane changing behaviour for accepted/rejected gaps, and to estimate speed/journey time benefits from compliance with lane use regulations. Analysis is being undertaken primarily through the use of logistic models to formulate the probability of an event occurring according to a range of inputs [7]. 70
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Figure 3: A ‘spiral function’ of following distance vs relative speed.
2.2 Driver Behaviour in an Emergency. Emergency deceleration situations are comparatively rare, and it would be difficult to obtain an adequate sample from on-road tests. Therefore, track and simulator based studies are to be undertaken to investigate the reaction of drivers to emergencies, as this will enable controlled, repeatable measurements in safe conditions. The findings should greatly increase the ability to model the formation of shockwaves, some of which may be triggered by 'rare' events such as these. The model will also be key to the definition of driver support systems, in which an understanding of driver braking behaviour will enable better system definition, ie. ones that will intervene at the 'correct time' (not so early as to generate a high level of false alarms). Three investigations are to be undertaken in the latter half of 2001: i) Firstly a small number of subjects will take part in following experiments on a test track, with the lead vehicle being a ‘surrogate’ (a fibreglass rear vehicle body towed by another vehicle. The use of such a ‘target’ minimises potential risks associated with any collisions that may occur). In addition to a standard following process, each subject will be exposed to a number of ‘high g’ deceleration
ii)
iii)
situations, in which their reactions in an emergency may be observed. This experiment will be re-performed using a simulator in order to allow a comparison to be made between empirical and ‘virtual’ data in this area. Secondly, a simulator experiment will be performed in which the driver is aided by a driver assistance system such as an ACC or Collision Warning device. Driver behaviour will be observed when they have to intervene and control the vehicle manually. The study will consider, for example, potential points of confusion and lapses in attention caused by over reliance or increases in reaction times caused by skill atrophy. A key issue is the driver’s perception of when intervention is required. (Earlier studies [8] have revealed a potential hazard when expectations of system performance exceed those that would be provided in practice). Lastly, a study will be performed to assess drivers perception of risk while not in control of a vehicle by exposing the simulator subjects to an AHS (Automated Highway System)/Platooning device, which will allow the maintenance of very close headways without human intervention. An understanding of the ideal and minimum headways that a driver is prepared to tolerate will allow a better understanding of the user needs of such systems
2.3 Fuzzy Modelling of Motorway Junctions. The third study of the project will assemble and calibrate a new junction module for the FLOWSIM microscopic simulation model (Fig 4). Although several approaches exist for the microscopic modelling of junctions, these are deterministic and do not incorporate error in driver perception. The use of a 'fuzzy' formulation will allow a more transparent model to be formulated, which can be transferred between differing junction geometric types more easily, and is also more straightforward to calibrate. The fuzzy formulation for car following and lane changing used in the FLOWSIM model has been proven as being more accurate than other models
available [9]. Such a tool will allow a new generation of control measures to be investigated, such as 'intelligent intersections', as well as the interaction of AVCSS systems with merge/demerge traffic.
Figure 4. A screen dump of the FLOWSIM simulation model. The study will focus on isolating those factors important for the merge/demerge decision, which may relate to gap size, relative speeds, distance remaining until onset of slip lane/termination of merge lane, overall density of traffic, and vehicle type. This study will be undertaken by examining both gaps accepted by a number of subjects during a test drive in the instrumented vehicle, and also by asking drivers to state at what point they would have commenced their merge at a number of junctions where no demerge was made. Additionally, some video based junction studies will be undertaken in order to enlarge the size of the database available. Once incorporated into the FLOWSIM model, a validation stage will be undertaken by comparing the results with those from a merge capacity database collected as part of an earlier study [10].
2.4 Next Generation AVCSS. The successful completion of the above three phases will allow initial investigations to begin on the definition of novel AVCSS. These may build on current advances such as ACC [11]. This study will
define the likely form of the next steps that can be taken in product design. These include: • Front and back ACC and CW, a system that controls the motion of a vehicle not only according to the position and relative speed of the vehicle in front, but also to any vehicle approaching from the rear, replicating natural behaviour where the risk of collision is 'spread' between vehicles to the front and rear. • Long range speed adaptation, where vehicle to vehicle communication may be used to pass information on vehicle 'state' in order to allow drivers (or a control system) to anticipate the arrival of a shockwave, thus 'smoothing' the traffic in a manner very similar to variable speed limits, but without the need for expensive infrastructure investment. Assessment of the impact of these measures will be made at several levels. These will include microscopic (changes in lane change gap acceptance, time to collision, and headway distributions) through the more macroscopic (speed-flow, lane densities and lane changing rate), which may be assessed through the FLOWSIM model. Also changes in collision and injury accident probabilities will be studied. Initial investigation of these issues have indicated that: many of the 'warning thresholds' designed for use in prototype CAS systems nearing market readiness in Japan may be using inappropriate thresholds and that the impact of ACC as proposed as a congestion reducing aid may be much less than that initially envisaged [11]: For simple cooperative platooning systems, throughput may show significant benefits [12] but in some circumstances substantial delays may be caused to merging and non equipped traffic: The use of inappropriate settings for ACC may reduce collision probability for the equipped vehicle, but may increase it for following vehicles [8].
2.5 Implications to Flow Modelling and Theory. In recent years there has been a renewed interest in continuum modelling, and several competing models have been proposed based on the original kinetic gas
models of Prigogine and Andrews [13]. The advantage such a model provides over traditional macroscopic, and indeed microscopic, models is rapidly becoming apparent. Theoretically, it may model much larger networks than would otherwise be the case. However, little calibration has been undertaken of the underlying equations of this approach, the kernal of which is based on simplified probabilistic models of microscopic behaviour. There is clearly a potential to incorporate the models from earlier phases of this project to strengthen the models, as well as parallel instrumented vehicle work already undertaken. In this phase an assessment will be undertaken of how successful work in this area could be, and the modelling opportunities it could afford. Although at present highly theoretical, one such model in Germany has recently been incorporated into a macroscopic motorway network prediction simulation model by Daimler Benz, which is aimed at Infrastructure and Network Operators [14].
3. Summary and Conclusion. During the course of the Motorway Operation Platform grant project, research will be undertaken in five areas vital to progressing our understanding of motorway operations. These studies are to be groundbreaking and to indicate specific further research needs. It is important that the findings are actively disseminated in order to highlight the importance of fundamental scientific work in this area and to: i) Bring about an increased understanding of driver behaviour, reaction to hazards and ITS and in-vehicle information. ii) Produce a set of validated simulation tools that may be further used in continued research on many ITS strategies. iii) Disseminate knowledge of the likely impacts and sensitivities of many ITS systems currently under development and the potential benefit available from many ideas currently at the theoretical stage.
Acknowledgements Work reported in this paper has been funded by the EPSRC in the U.K. under contract number GR/M94410. Support for the instrumented vehicle continues to be provided by TRW-Lucas and the University of Southampton. Further details on this project may be obtained at: http://www.soton.ac.uk/~trgwww/research/platform/platform.htm
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