Exploration of the impact of Intelligent Speed Adaptation and Co-operative Following and Merging on Highways using MIXIC

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1 TNO-report Inro/NK2003- TNO Inro Exploration of the impact of Intelligent Speed Adaptation and Co-operative Following and Merging on Highways using MIXIC Schoemakerstraat 97 P.O. Box JA Delft The Netherlands Phone Fax Internet: Contactperson H. Schuurman (AVV) C.M.J. Tampère (TNO Inro) All rights reserved. No part of this publication may be Reproduced and/or published by print, photoprint, microfilm or any other means without the previous written consent of TNO. In cast this report was drafted on Instructions, the rights and obligations of contracting parties are subject to either the Standard Conditions for Research Instructions given to TNO, or the relevant agreement concluded between the contracting parties. Submitting the report for inspection to Parties who have a direct interest is permitted. 2003TNO Date November 1999 Number 99/NK/162 Department of Traffic Author(s) C.M.J. Tampère J.H. Hogema R.T. van Katwijk B. van Arem TNO Inro carries out research and offers consultancy services in the field of infrastructure, transport and regional development, with the aim of strengthening regional competitive power. Netherlands organization for applied scientific research

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3 99/NK/162 i PREFACE This report describes the results of an exploratory study into the impacts of Intelligent Speed Adaptation and Co-operative Following and Merging on traffic flows on highways. The study was conducted using the microscopic simulation model MIXIC. This study was commissioned by the Transport Research Centre (AVV) of the Dutch Ministry of Transport, Public Works and Water Management. From October 1998 until November 1999 these people constituted the project team: Jeroen Hogema (TNO Human Factors Research Institute) Bart van Arem, Ronald van Katwijk & Chris Tampère (TNO Inro, main contractor) On behalf of AVV Henk Schuurman participated in the project. An important input of expert knowledge in the project was contributed by: Martie van der Vlist (TNO Inro), Alexander de Vos (TNO Human Factor Research Institute), Frank Waaldijk (AVV) and Joost Zuurbier (TNO Automotive). Delft, November 1999 Chris Tampère

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5 99/NK/162 iii ABSTRACT This study explores how and to what extent communication between vehicles or between vehicle and infrastructure can contribute to more stable traffic flow for better throughput and safer driving on highways. For this purpose the desired functionality of 3 systems has been determined: Intelligent Speed adaptation, Co-operative Following and Co-operative Merging. The former two have been implemented in the microscopic simulation model MIXIC and were used in an exploratory simulation experiment. The simulation of CF led to the identification of those elements in the design of the CF system that are crucial for adequate functioning: proper management and processing of different warning messages and integration of different functionalities into one single driver assistance system. The explorative simulation of ISA in traffic flows shows a decrease in throughput (volume) combined with positive effects on safety (shock waves, speed variance) when ISA is introduced. For the validation of these results more research into the driver s behaviour in the presence of ISA is recommendable.

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7 99/NK/162 v KORTE SAMENVATTING Dit rapport brengt verslag uit van een studie waarin werd verkend op welke wijze en in hoeverre de communicatie tussen voertuigen onderling of tussen voertuigen en een wegkant systeem kan bijdragen aan een betere en veiligere verkeersafwikkeling op autosnelwegen. Hiertoe werd van 3 systemen de gewenste functionaliteit vastgesteld: Intelligent Speed adaptation (ISA), Co-operative Following (CF) en Co-operative Merging (CM). De twee eerste systemen werden vervolgens in het microscopisch simulatiemodel MIXIC geïmplementeerd en gebruikt in verkennende simulaties. De simulatie van CF droeg in belangrijke mate bij aan de identificatie van en inzicht in die elementen in het ontwerp van CF die bepalend zijn voor een optimale werking: adequaat management van verschillende waarschuwingsberichten en integratie van de verschillende deelfunctionaliteiten in één geïntegreerd bestuurders ondersteuningssysteem. De verkennende simulaties van ISA vertonen een afname van de doorstroming (intensiteiten) en een verbetering van de verkeersveiligheid (schokgolven, snelheidsverschillen) met ISA. Aanvullend onderzoek met het oog op inzicht in bestuurdersgedrag in aanwezigheid van ISA is wenselijk om de resultaten te kunnen valideren.

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9 99/NK/162 vii SUMMARY This study explores how and to what extent communication between vehicles or between vehicle and infrastructure can contribute to more stable traffic flow for better throughput and safer driving on highways. For this purpose a literature survey was conducted and experts were consulted resulting in a definition of the desired functionality of 3 candidate systems:?? Co-operative Following (CF): automated longitudinal control combined with inter-vehicle communication allows for anticipation to severe braking manoeuvres in emerging shock waves with the aim of smoothening traffic flow and enhancing traffic safety.?? Co-operative Merging (CM): automated longitudinal control combined with inter-vehicle communication assists the driver in lane changing manoeuvres by creating and maintaining an appropriate gap in the target lane. This systems aims at smoothening traffic flow by preventing disturbances due to lane changing.?? Intelligent Speed Adaptation (ISA): communication between car and infrastructure is used for external speed control with the aim of smoother traffic flow due to more homogeneous driving behaviour. The functionality of CF and ISA have been elaborated in functional specifications for the microscopic simulation model MIXIC and the model was extended with these functionalities. Co-operative Following The first simulation results with CF in a well controlled experiment with a platoon of mixed CF- and non-equipped traffic on a single lane show the potential of the system. But the preliminary test results in mixed traffic on more lanes revealed that the functional design still has undesirable effects in these complex circumstances. The potential advantages of CF with respect to throughput and traffic safety could therefore not be confirmed. However the simulation results with CF are an important contribution in the identification of those elements in the functional design of this new system that determine the well-functioning of cooperative following behaviour in mixed traffic:?? the nature of the CF-controller should prevent excessive response to new warnings and have adequate management of speed warnings coming from different vehicles and times;?? different driver support systems such as Adaptive Cruise Control and CF logic should be integrated in one controller so that continuous and stable control is guaranteed especially when switching between different functionalities.

10 viii 99/NK/162 Intelligent Speed Adapta tion The effect of ISA system was explored in a simulation of traffic on a highway with a bottleneck due to a lane drop. Criteria for throughput and safety were analysed with gradually increasing traffic demand consisting of traffic with 10%, 20%, 30%, 40% and 0% penetration level of ISA in passenger cars until capacity was reached. The ISA system had a restrictive nature, i.e. drivers can by no means exceed the current speed limit transmitted by the roadside beacons. This speed limit was set by an algorithm based on measurement of the occupancy upstream of the bottleneck. The results of the simulation clearly show the homogenisation of traffic that can be expected after the introduction of ISA: speed variance of vehicles within one lane as well as speed difference between lanes decreases. Also the division over lanes slightly shifts toward equally balanced traffic load. The effect on throughput is surprising: apart from the expected decrease in average vehicle speed also the average traffic volume as well as the maximally observed traffic volume (-7%) in the simulation decreases when ISA is introduced in more vehicles. However, these results should be interpreted carefully since it can not be determined without further research whether this effect is inherent to the use of this type of ISA system on a highway or whether the decreasing volumes should be contributed to limitations of the mandatory lane change model of MIXIC. This model is of crucial importance with respect to throughput of a bottleneck but was designed for non-equipped traffic. With respect to traffic safety the simulations show a decrease of the number of vehicles that are involved in severe shock waves during the merging process. Together with the already mentioned lower variance in speeds within and between lanes these criteria indicate that the introduction of ISA has a positive effect on traffic safety.

11 99/NK/162 ix SAMENVATTING Dit rapport brengt verslag uit van een studie waarin werd verkend op welke wijze en in hoeverre de communicatie tussen voertuigen onderling of tussen voertuigen en een wegkant systeem kan bijdragen aan een betere en veiligere verkeersafwikkeling op autosnelwegen. Hiertoe werd een literatuurscan uitgevoerd en werd tijdens een workshop met experts op het gebied van verkeersafwikkeling en bestuurders ondersteunende systemen de gewenste functionaliteit vastgesteld van een drietal systemen, te weten:?? Co-operative Following (CF): hier wordt geautomatiseerde longitudinale controle van het voertuig gecombineerd met communicatie tussen voertuigen onderling zodat kan geanticipeerd worden op sterke remmanoeuvres in schokgolven. Dit kan bijdragen aan een vloeiender verkeersafwikkeling ten behoeve van een betere doorstroming en een grotere verkeersveiligheid.?? Co-operative Merging (CM): hier wordt geautomatiseerde longitudinale controle van het voertuig gecombineerd met communicatie tussen voertuigen onderling ter ondersteuning van invoegmanoeuvres. Voertuigen creëren in onderlinge samenwerking een geschikte hiaat om in te voegen. Door het vermijden van bruusk invoeggedrag kan dit systeem bijdragen aan een vloeiender verkeersafwikkeling ten behoeve van een betere doorstroming en een grotere verkeersveiligheid.?? Intelligent Speed Adaptation (ISA): hier ligt communicatie tussen een wegkantsysteem en het voertuig aan de basis. Het wegkantsysteem bepaalt aan de hand van metingen een optimale snelheid en communiceert deze aan een snelheidsbegrenzer in het voertuig. Dit systeem kan bijdragen aan een homogener verkeersbeeld ten behoeve van een betere doorstroming en een grotere verkeersveiligheid. Vervolgens werden het CF en het ISA systeem verder uitgewerkt tot functionele specificaties voor het microscopisch verkeerssimulatie model MIXIC en geïmplementeerd. Co-operative Following De eerste simulatieresultaten met CF in een gecontroleerd experiment met een beperkt aantal voertuigen op één strook tonen aan dat het systeem in principe werkt zoals verwacht, maar tests in een gemengde verkeersstroom op een meerstrooks-autosnelweg brachten aan het licht dat het functioneel ontwerp in die omstandigheden nog niet het gewenste effect vertoont. Hierdoor konden de potentiële voordelen voor de doorstroming en de verkeersveiligheid van dit systeem niet worden onderzocht. Deze testresultaten hebben echter in bela ngrijke mate bijgedragen aan het identificeren van die elementen in het functioneel ontwerp van dit nieuwe concept die bepalend zijn voor het goed functioneren van coöperatief volggedrag in een gemengde verkeersstroom van voertuigen met en zonder het systeem:

12 x 99/NK/162?? de eigenschappen van de CF-regelaar moeten vermijden dat het voertuig té sterk reageert op nieuwe berichten en er moet voldoende zorg besteed worden aan een adequaat afwegen van het belang van verschillende berichten verzonden door verschillende voertuigen eventueel op verschillende tijdstippen.?? verschillende bestuurder ondersteunende deelsystemen zoals Cruise Control, Adaptive Cruise Control en Co-operative Following moeten in een regelaar geïntegreerd worden om te vermijden dat systemen elkaar tegenwerken en om te garanderen dat de overgang tussen de verschillende functionaliteiten geleidelijk en logisch gebeurt. Intelligent Speed Adaptation Het ISA systeem werd gesimuleerd in een toepassing bij een wegversmalling van 3 naar 2 stroken op een snelweg. Bij een geleidelijk toenemende drukte werden criteria met betrekking tot doorstroming en veiligheid geanalyseerd voor verkeer waarvan 10%, 20%, 30%, 40% of 0% van de personenwagens was uitgerust met het ISA systeem. Dit systeem was beperkend van aard d.w.z. dat de bestuurder op generlei wijze de heersende snelheidslimiet kan overschrijden. Deze snelheid wordt door een wegkantsysteem bepaald aan de hand van metingen van de bezettingsgraad stroomopwaarts van de versmalling en door bakens langs de kant van de weg aan de voertuigen gecommuniceerd. De resultaten van deze verkennende simulatie tonen het homogeniserende effect dat van ISA verwacht kan worden: dit komt tot uiting in een kleinere variantie van de snelheden tussen stroken alsook van voertuigen binnen eenzelfde strook. Ook de verdeling van verkeer over de stroken vertoont een lichte verschuiving naar een gelijkmatige verdeling. Het effect op de doorstroming is enigszins verrassend: zoals verwacht daalt de gemiddelde snelheid van het verkeer, maar ook voor de intensiteit, zowel gemiddeld als maximaal (-7%), worden met de graduele introductie van ISA in de simulatie lagere waarden gemeten. Deze resultaten moeten met de nodige omzichtigheid geïnterpreteerd worden omdat zonder diepgaander onderzoek niet uitgemaakt kan worden of de lagere intensiteiten echt inherent zijn aan het gebruik van dit type ISA systeem of dat ze te wijten zijn aan beperkingen in het gedwongen rijstrookwisselmodel van MIXIC in combinatie met een snelheidsbegrenzer. Dit deelmodel heeft immers een bepalende invloed op de doorstroming van een versmalling en werd gevalideerd voor normaal verkeer. Mogelijke gedragswijzigingen ten gevolge van een snelheidsbegrenzer zijn niet in het model opgenomen. Wat betreft veiligheid voorspelt het model dat er tijdens het invoegproces voor de versmalling minder voertuigen betrokken zijn in sterke schokgolven. Samen met de kleinere snelheidsverschillen tussen stroken maar vooral binnen een zelfde strook volgt dus uit deze verkenning dat ISA de verkeersveiligheid bevordert.

13 TABLE OF CONTENTS page Preface... i Abstract... iii Korte samenvatting... v Summary... vii Samenvatting... ix 1 Introduction Background Goal of the research project Outline and content of the project Functionality of Co-operative Following, Co-operative Merging and Intelligent Speed Adaptation Literature survey Functional design of the Co-operative Following system Functional design of the Co-operative Merging system Functional design of the Intelligent Speed Adaptation system Experimental set-up and simulation results for the Co-operative Following system Set-up of the CF simulation CF simulation results Discussion of the CF simulation Experimental set-up and simulation results for the Intelligent Speed Adaptation system Set-up of the ISA simulation ISA Simulation results Discussion of the ISA simulation Conclusions and recommendations Conclusions Outlook for further study References... 43

14 Appendix A: Functional specification of Co-operative Following in MIXIC... 4 A.1 Introduction: system philosophy... 4 A.2 Inter vehicle communication... 4 A.3 Processing of received data A.4 Vehicle behaviour A. Driver behaviour... 0 Appendix B: Functional specification of Intelligent Speed Adaptation in MIXIC... 1 B.1 Introduction... 1 B.2 Functional design of the In-car system... 2 B.3 In-vehicle Intelligent Speed Adapter... 3 B.4 ISA Road-side system... 6 B. Driver-ISA interaction Appendix C: New Vehicle Model with Engine Braking C.1 Changes to the vehicle model C.2 Changes to the driver model Appendix D: Detailed simulation results D.1 Speed Volume Density Time relationships per variant D.2 Average Speeds and Traffic Volumes D.3 Removed Vehicles and Shock wave information... 83

15 99/NK/ INTRODUCTION 1.1 Background Systems of Automated Vehicle Guidance (AVG) are no longer a far away future dream of automotive engineers. AVG-systems are automated systems that support or (partly) take over the driver task. The introduction on our motorways of the Adaptive Cruise Control (ACC) in the late nineties is another step towards partly human partly computer controlled driving. The system is a variant of classical cruise control that not only maintains a driver-set speed but also automatically adapts the speed when approaching a predecessor in order to preserve a safe and comfortable distance. This evolution was anticipated in the early nineties by both TNO and the Transport Research Centre of the Ministry of Transport, Public Works and Water management, who sensed the future need for analysis tools capable of examining the impact of partly automated driving. In a joint effort they developed the microscopic traffic simulation model MIXIC. This model contains a detailed (microscopic) driver and vehicle model, both based on extensive research and knowledge of driver and vehicle behaviour available in different TNO institutes (TNO Human Factors Research Institute, TNO Automotive, TNO Inro, TNO Institute of Applied Physics) resulting in realistic macroscopic traffic flow on a motorway link. MIXIC was specifically designed for analysis of the impact of automated systems, which partly take over driver tasks. The first application of MIXIC was ACC. Several studies involving ACC were conducted (Van Arem et al, 199; Van Arem et al., 1997a; Van Arem et al.; Hogema & Van der Horst, 1998). 1.2 Goal of the research project The main findings of the different analyses of ACC with MIXIC can briefly be summarised as follows: ACC can contribute significantly to the stability of traffic flow and can affect traffic performance (capacity) in a bottleneck situation both positively and negatively. The latter depends on penetration level and parameter choice of the ACC controller. The introduction of a special lane for ACC vehicles has a similar effect but introduces disturbances in the zone were traffic split-ups. The question that lead to the new developments reported in the underlying document is: How and to what extent can communication between vehicles or between vehicle and infrastructure contribute to more stable traffic flow for better throughput and safer driving on highways? Two AVG-concepts seem promising with respect to this: 1. Co-operative Following and Merging (CFM): for these systems inter-vehicle communication is the basis of co-operative automated longitudinal control during manoeuvring. These imply manoeuvres like braking during car-following in the own lane (Co-operative Following or CF) as well as manoeuvres during lane changing (Co-operative Merging or CM)

16 2 99/NK/ Intelligent Speed Adaptation (ISA): communication between car and infrastructure for external speed control is the basis of homogeneous driving behaviour with the aim of smoother traffic flow. 1.3 Outline and content of the project The project was conducted in three phases: 1. functional specification of ISA and CFM 2. implementation and testing of ISA and CFM in MIXIC 3. exploration of the effects of ISA and CFM on traffic flow The first phase started with an orientational literature survey into the possible functionality of AVGsystems based on inter-vehicle communication or on communication between the vehicle and the infrastructure for external speed control. The findings of this survey were then discussed in a workshop with experts in the field of Automated Vehicle Guidance, driver behaviour and traffic flow on highways. Their task was to identify the main features that should be included in the functional design of a Co-operative Following, Co-operative Merging and Intelligent Speed Adaptation system for attaining optimal throughput on a highway while maintaining (or even improving) traffic safety. A global functional description of CF, CM and ISA was extracted and is reported in chapter 2 of this report. Since the largest potential for improvement of traffic flow is expected from CF and ISA these two systems were selected for further research within the project. The functionality of CF and ISA was elaborated in more detail for implementation in MIXIC. The specifications for CF and ISA in MIXIC are reported in chapter 2 and in appendices A, B and C of this report. In the second phase these specifications were implemented and tested as extensions to MIXIC 1.3. The extended simulation model was prepared for analysis of the CF and ISA system in the third phase. In the third phase of this project the impacts of CF and ISA on traffic flow were explored using the MIXIC simulation. From a first observation of the impact of the CF system it could be concluded that it was more interesting to first investigate the CF controller and its interference with other controllers (such as the ACC controller and the human driver) before conducting larger simulation experiments. The study of CF was therefore focussed on gaining more insight into these issues. This is discussed in chapter 3. The ISA system was used in an exploratory simulation experiment. The set-up and the results of this experiment are described in chapter 4. Finally in chapter of this report the conclusions of this exploratory study are summarised and some recommendations for the continuation of this research are formulated.

17 99/NK/ FUNCTIONALITY OF CO-OPERATIVE FOLLOWING, CO-OPERATIVE MERGING AND INTELLIGENT SPEED ADAPTATION The inter-vehicle communication in a Co-operative Following and Merging system serves two different purposes:?? Co-ordination of longitudinal manoeuvring, such as co-ordinated accelerating or decelerating; and?? Co-ordination of lateral manoeuvring, such as lane changing, weaving and merging. In the remainder of this report these are treated separately. Vehicles can be equipped with a Cooperative Following system (CF), a Co-operative Merging system (CM) or a mixture of both. In this chapter the results of a literature survey into the functionality of CF, CM and ISA are summarised in paragraph 2.1. The complete literature survey can be found in Tampère and Hogema (1999). In paragraphs 2.2, 2.3 and 2.4 the functionality of the three systems is argumented and described. 2.1 Literature survey The aim of the literature survey was to mark out the functionality of Co-operative Following and Merging and Intelligent Speed Adaptation so that the positive impact on traffic flow is maximised. Positive impact was defined as favourable for both traffic safety and throughput. The benefits on throughput were estimated and analysed for a bottleneck situation caused by a lane drop. This is a reference situation which was also used in previous MIXIC studies (Van Arem et al., 1997a) Co-operative Following (CF) The principle of the CF-concept as a safety and throughput promoting system is: optimisation of the car-following process. Examples were found in different degrees of co-ordination. The concept of free agent vehicles preserves autonomous vehicle operation, based on both autonomously and co-operatively gathered information. The concept is described among others by Kanaris et al. (in: Ioannou, 1997). The car-following with respect to the immediate predecessor builds on drivers perception and the own sensors (e.g. ACC sensor). The driver and the assistant system (e.g. CF controller) obtain additional information about the current traffic conditions such as sudden braking ahead. For autoplatooning as presented by Ward (in: Ioannou, 1997) the inter-vehicle communication is used in normal traffic conditions to interchange information on prevailing traffic conditions (average speeds, headways) around the transmitting car. When the system recognises that traffic comes near to capacity (and hence: breakdown) the equipped vehicles look up each others presence and intensify the

18 4 99/NK/162 communication which from then serves as an electronic tow-bar. The operation of the first vehicle (braking, accelerating, maybe steering as well) is then immediately taken over by the following cars. This allows driving with short headways and therefore creates new space on the highway and better throughput. Where the previous systems operate in mixed equipped and non-equipped traffic the concept of full platooning, described by Ioannou (in: Ioannou, 1997) requires separate infrastructure (like dedicated separate lanes). Here also the vehicles are linked by electronic tow-bars so that short headways are possible and manoeuvres are initiated by one vehicle and executed simultaneously by all platooning vehicles. High capacity values of up to 800 veh/h per lane can be achieved. In order to be able to optimally fill a special lane for platooning vehicles the system should be combined with some kind of co-operative system for merging into the platoon as well as for leaving it Co-operative Merging (CM) The principle of the CM-concept as a safety and throughput promoting system is: optimisation of the merging process. By smoothening merging manoeuvres the emergence of shock waves can be prevented, which might have a stabilising effect. Examples that support lane changing in mixed traffic were not found. However, many authors address the merging process at access roads and introduce optimisation strategies using inter-vehicle communication. For this purpose different degrees of coordination are possible. Gibson (199) and Ran (1996) describe a process in which vehicles in adjacent lanes select a virtual predecessor. They adapt their longitudinal following behaviour to both the real and the virtual predecessor. The vehicle in the target lane yields and leaves an appropriate gap. The merging vehicle aligns to the gap, adapting his speed and positioning to its new position in the target lane. Intervehicle communication is needed for selecting a candidate gap in the target lane and thereafter for tracking the manoeuvre and cancelling it when necessary. Gibson (199) and Ran (1996) also present a more co-operative variant: co-operative intelligence. Here the merging vehicle uses vehicle infrastructure communication to announce the intended merge manoeuvre. The infrastructure monitors headways in the upcoming traffic and selects an appropriate gap in the target lane. The vehicles involved are then instructed to maintain or if necessary to enlarge this gap by the time the merging vehicle comes alongside of the target lane. The yield and align steps in the aforementioned process have then already been carried out and the merging vehicle can immediately take its place in the target lane. Gibson (199) also presents a variant of this process for full platooning concepts. The merging manoeuvre is basically the same except for the selection of the gap and the yielding process. The platoons are cruising on separate infrastructure with safe inter-platoon distances. The inter-platoon

19 99/NK/162 headway is large enough to allow for a new vehicle to merge into the target lane which makes yielding unnecessary. The merging of the new vehicle increases the platoon size, possibly necessitating the platoon to split up. Appropriate scenarios have been elaborated for this manoeuvre Intelligent Speed Adaptation (ISA) Specifying the functionality of an ISA system requires a choice of an in-car system combined with an ISA-control algorithm. Both can vary and different combinations of in-car systems and algorithms are possible. The in-car system (e.g. Veenbaas & Oostenbrink, 1996) is characterised by the extent to which it is possible for a driver to overrule the speed limiter. The extent to which a driver follows up the speed limit is called: compliance level. The compliance can be influenced by the in-car system, ranging from informing systems over supporting systems to restrictive systems. An informing in-car system only displays the current speed limit e.g. on a dedicated display or a LED on the speedometer. A supporting in-car system encourages the driver not to exceed the speed limit. An increasingly loud acoustic signal or increasingly strong haptive feed-back (counterpressure) on the accelerator pedal warns the driver in case he drives too fast. The restrictive in-car system strictly limits the accelerating capabilities of the car once the speed limit is reached or exceeded. The limit can be enforced passively by controlling the gas throttle so that the engine brake reduces the speed to an allowable level or actively by electronic control of the brakes. Hogema and Van der Horst (1998) analysed a variant of ACC where the reference speed was set by ISA-beacons. This can be seen as an ISA in-car system. The system could easily be overruled by switching the ACC off. The ISA-control algorithm is the road-side system that sets the ISA in-car speed limit based on actual knowledge of prevailing traffic conditions. For the control algorithm inspiration can be found in speed homogenisation and other variable speed limit algorithms. In these applications the speed limit is usually set by variable message signs (VMS) instead of beacons. The logic behind the control algorithm can be either following or anticipating. A following or reactive strategy (AGV, 1997) monitors the traffic conditions for instance in a bottleneck. When measurements (speed, volume) indicate that traffic flow breaks down the speed limit for upcoming traffic is adapted appropriately. The effects of reactive algorithms can be significant on traffic safety since the measure warns drivers of problems ahead and reduces the speed and speed variance (homogenisation) especially when dense and unstable traffic conditions can be expected. Improvement in throughput is not expected due to the reactive nature of the measure. An anticipating algorithm tries to overcome this limitation. The aim is to prevent traffic in the bottleneck from breaking down by reducing the speed limit earlier. The speed is therefore limited to a level by which the upcoming traffic can normally be processed without breakdown. Examples of this

20 6 99/NK/162 category were not found. The idea of anticipation will be further elaborated in paragraph based on the analogy with ramp metering algorithms. 2.2 Functional design of the Co-operative Following system The rationale behind Co-operative Following or Merging systems as safety and throughput improvement systems is the limitation of the impact of shock waves. Indeed, the emergence of shock waves in traffic flow affects both traffic safety and efficiency of traffic flow: the strong decelerations combined with short headways in a shock wave can cause collisions; on the other hand the emergence of a shock wave can be responsible for the transition from (meta-) stable highly efficient traffic flow to unstable inefficient traffic flow. A co-operative following strategy aims at damping shock waves once they have appeared. Normally a shock wave is generated by the leader in a platoon with short headways. The shock wave is amplified when the reaction of the following driver to the braking manoeuvre is delayed so that the follower has to brake harder than the leader to avoid a collision. This can cause his follower to brake even harder etc. A driver in a platoon reacts to the shock wave after a time delay which equals the sum of the reaction times of himself and all his predecessors in the shock wave. Damping of the shock wave is done by eliminating this time delay. This can be done by communicating the intended braking manoeuvre by the leader to all the following vehicles who can then react simultaneously instead of consecutively. CF-equipped cars who have to brake hard transmit their actual speed to the upcoming cars. Those equipped with CF receive the warning and start responding to it immediately while non-equipped cars are not yet aware of the emerging shock wave. The effect of co-operative following compared to autonomous following behaviour such as human driving or ACC is presented in figure 2.1. Autonomous driving (in car-following situation) can be modelled by a spring and a damper. The spring models the pure distance keeping skill: when the headway becomes too small the spring will exert a repulsive force or deceleration. Large headways will cause a pulling force or acceleration. The damper models the collision avoidance skill which aims at minimising speed differences with the predecessor: positive speed differences cause a repulsive force or deceleration and vice versa. The functioning of a CF connection can be modelled as an additional damper between the transmitting car and the receiving car.

21 99/NK/ k AICC k human k AICC k AICC k human AICC C AICC Human C human AICC C AICC AICC C AICC Human C human Human Autonomous Following 6 AICC k AICC C AICC k human Human C human 4 AICC k AICC C AICC 3 AICC k AICC C AICC 2 k 1 human Human C human Human DCF D CF 6 k CF k human k CF k AICC k human CF C CF Human C human CF C AICC Human C CF CF C AICC human DCF Co-operative Following D CF 6 3 k CF k human k k 1 CF AICC khuman CF C Human CF C human CF C AICC C Human CF CF AICC C human Figure 2.1: Possible impact of CF in shock waves. Figure 2.1 shows the positive impact that can be expected from this additional damper: due to their immediate reaction the CF equipped cars spread their braking manoeuvre over a longer time period thus allowing smaller decele rations. This will damp the shock wave. Unlike the situation in figure 2.1 there will be more than one transmitting vehicle. The receiving CFvehicle then has to determine which warning is the more relevant one. Both time and space aspects determine the relevancy of a warning message. When a warning is received the reaction to this warning will last longer than the duration of transmission of the message (which is only a few m/sec). Therefore the message is stored by the receiving vehicle together with the time of reception and the vehicle will consider the warning in its braking/accelerating decisions during some time. The older the warning the less relevant it gets and after a while it can be discarded. Several warnings sent by vehicles on different relative positions can reach a receiving vehicle at a time. Messages coming from further away will then be considered less relevant than warnings from the immediate predecessors. The principles of the transmission, the judgement of relevancy of incoming warnings and the processing of the speed warnings by the CF-controller (which eventually determines the actual acceleration) are further elaborated and turned into functional specifications for the microscopic simulation model MIXIC in appendix A.

22 8 99/NK/ Functional design of the Co-operative Merging system Just like Co-operative Following the rationale behind Co-operative Merging as safety and throughput improvement system is the limitation of the impact of shock waves. However, where CF aimed at limiting the progression and severity of shock waves by damping them, the CM strategy tries to avoid the initiation of shock waves during lane change manoeuvres. The CM-system described here uses the same principle of the virtual predecessor as was reported by Gibson (199) and Ran (1996). The CM-system differs in that it is meant for use in mixed traffic. This implies that CM-equipped vehicles must be able to find each other in situations where a lane change is mandatory. Therefore the CM-vehicle communicates its intention to the vehicles in the target lane. When another CM-vehicle is driving within a certain range it responds to the request and starts creating a gap for the merging vehicle. For this yielding manoeuvre the vehicles have an ACClike controller that is able to respond to both communicated positions and radar sensors. This CMcontroller gradually changes from car-following within the own lane to car-following in both the own and the adjacent lane: for the vehicle in the target lane the controller gradually takes the merging candidate as a virtual predecessor and the merging vehicle gradually takes the predecessor of this yielding vehicle as virtual predecessor. This process is visualised in figure 2.2. Human CM AICC Human AICC CM human CM : merging CM-vehicle : link to predecessor in own lane : yielding CM-vehicle : link to virtual predecessor Figure 2.2: Principle of Co-operative Merging During this gradual take-over the yielding and align to gap processes take place simultaneously after which the lane change takes place. The concepts of CF, CM and ISA have been conceptually defined and have to be elaborated into functional specifications. Although all three systems are promising (either separately or in combinations) only two systems could be further detailed within the scope of this project. The Cooperative Following and the Intelligent Speed Adaptation systems were chosen. The main reason is

23 99/NK/162 9 that the potential impact of these systems is expected to be larger because they affect the vehicle s behaviour in a wider range of situations: while CM only avoids shock waves caused by (mandatory) lane changes CF damps out every shock wave regardless of its origin and ISA limits the speed regardless of the vehicle s actual manoeuvre. However it is an interesting subject for further research to elaborate the idea of CM and explore its impact in specific situations like lane drops either separately or in combination with other supporting systems like ACC, CF or ISA. 2.4 Functional design of the Intelligent Speed Adaptation system The functional design of an ISA-system consists of a definition of the in-car system and of the roadside system (ISA-algorithm). These two subsystems are treated separately in the next paragraphs Functional design of the ISA in-car system The functional design of an ISA in-car system defines the level of compliance. Since this project examines the potential maximal impact of ISA on traffic flow the most interesting case is the restrictive one. The ISA controller calculates the gas pedal position that would be needed to accelerate to the ISAspeed limit. The actual gas throttle position that is fed into the vehicle model is the minimum of the gas pedal position that would be set by the driver in absence of any ISA system and the pedal position set by the ISA controller. Therefore the driver is overruled by the ISA controller when he tries to accelerate to speeds higher than the ISA speed limit. This feature reflects the restrictive nature of the controller. Note however that this specification can easily be adapted to a supporting version of the ISA controller. An additional module would therefore be necessary that weighs both gas pedal positions so that an intermediate gas pedal position is chosen. In this way a certain level of compliance is simulated. The functional design of the ISA controller is equal to that part of an ACC controller that maintains the reference speed. However, the ISA controller can only intervene through the position of the gas pedal and does not have a brake actuator. Smaller decelerations can be achieved by using the engine brake. An engine brake was not previously featured in MIXIC and was added to the model for this purpose. A brief description of the new engine model including engine braking is given in paragraph and more details can be found in appendix B where the ISA system is extensively specified.

24 10 99/NK/ The road-side system and ISA-algorithm The road-side system of the ISA system is closely related to the ISA-algorithm that is used. For this algorithm the actual traffic state should be monitored by one or more ISA-observers. The algorithm used in this study has an anticipating nature. It uses only one observer that is located upstream from the bottleneck where traffic flow should be smoothened by ISA. Two beacons are used: the first one is located upstream of the bottleneck and intends to set the speed limit when necessary; the second one is located downstream and switches the ISA system off again. The lay out of the road-side system is schematically presented in figure 2.3. Non-ISA drivers would in practice be informed about the current speed limit, e.g. through Variable Message Signs (VMS). However, for this study this was not modelled. In fact the simulations therefore represent a worst case scenario with a highly heterogeneous mixture of restricted ISAdrivers and non-informed normal drivers. This choice is motivated by the concern that the impact on the simulation would be solely attributed to ISA and not to the response of non-isa drivers to VMS. ISA 'on'- beacon ISA observer ISA 'off'- beacon Driving Direction L_B1 L_B0 L_MP1 Figure 2.3: Lay-out of the ISA road-side system The logic of the ISA-algorithm was inspired by existing algorithms for ramp metering in the Netherlands (Van Velsen & Stevens-van der Geer, 1996). An on-ramp is indeed comparable to a lane drop situation: the combined traffic demand of the main road and the additional lane of the on-ramp has to merge so that the combination of both can be processed by the main road alone when the acceleration lane drops off. With a normal lane-drop (e.g. figure 2.3) the traffic demand of the continued lanes and of the (additional) left lane has to merge so that the combination of both can be processed by the continued lanes alone after the lane-drop. Basically the aim of ramp metering is to adapt the traffic demand on the on-ramp so that total traffic demand never exceeds capacity of the bottleneck. For that purpose the density or intensity of upcoming traffic on the main road is measured, so that the capacity for assimilating additional traffic

25 99/NK/ can be determined. It is then assured that capacity is not exceeded by restricting traffic volume of the on-ramp and spreading it over time using traffic lights. And this is where the analogy ends. With ISA the traffic of both the continued lanes and the left lane is influenced. This is not done by adapting the traffic volume of any lane but by adapting the speeds. The logic behind speed adaptation can be explained on the basis of figure 2.4. This figure shows a typical relationship between the speed and the occupancy as measured in MIXIC. Occupancy of a point on a lane is defined as the percentage of time that the point is covered by traffic. The occupancy of a lane is closely related to the density but has the advantage that it is defined for a point on the road instead of along a certain distance and therefore it can easily be measured directly (e.g. by inductive loop detectors). The occupancy of a multi-lane cross-section is then defined as the arithmetical average occupancy of the lanes. Speed (km/h) V1 V2 O1 O2 0.0%.0% 10.0% 1.0% 20.0% Occupancy (%) Figure 2.4: Occupancy-Speed relationship of non-isa traffic on 2-lanes. The curve was obtained from a simulation in MIXIC with occupancy and speed aggregates per minute ( + markers). These measurements were then fitted to a linear curve. The occupancy-speed relationship of a cross-section is always decreasing in nature and hardly varies with the number of lanes in the cross-section. Suppose traffic with occupancy O 1 on a the 3-lane road (N 1 =3) approaches a lane-drop with speed V 1 (a point on the curve in figure 2.4). If the same volume wants to flow into the 2-lane section (N 2 =2) then density (and hence: occupancy) has to increase with a factor N 1 /N 2 = 3/2. The occupancy O 2 then equals 1. * O 1 and the speed V 2 with which the traffic can be processed on the 2-lane section can be read from figure 2.4. This is the principle of the ISA-algorithm. Referring to figure 2.3 the occupancy O 1 is measured by the ISA observer aggregated over a user-defined time interval (e.g. 1 minute). The ISA algorithm uses a parameterised Occupancy-Speed curve to calculate the speed V 2 according to the above mentioned

26 12 99/NK/162 principle. Before transmitting this speed V 2 to the traffic via the ISA on -beacon (see figure 2.3) it is smoothened to prevent that subsequent speed limits differ too much. For more details about the specification of the ISA-algorithm the reader is referred to appendix C.

27 99/NK/ EXPERIMENTAL SET-UP AND SIMULATION RESULTS FOR THE CO- OPERATIVE FOLLOWING SYSTEM 3.1 Set-up of the CF simulation The specifications of the CF-system were implemented in MIXIC and tested. From these first observations of the impact of the CF system it could be concluded that it was more interesting to first investigate the behaviour of the CF system in a simulation with a limited number of vehicles before conducting larger simulation experiments. The study of CF was therefore focussed on gaining more insight into the CF system and its controller. The small scale simulation consists of a platoon of mixed CF- and non-cf vehicles on a single lane road (see figure 3.1). The platoon originally driving at 120 km/h approaches a slowly driving truck (60 km/h). Once the platoon leader has spotted the truck he brakes suddenly. Since the platoon leader is a CF-vehicle he will transmit speed warnings if his deceleration exceeds a threshold (e.g. 2.0 m/s^2). Figure 3.1: Illustration of the Co-operative Following scenario in a platoon in MIXIC. The CF-vehicles in the platoon (blue) respond immediately to the warning message while the autonomously controlled cars (red; human drivers in this example) are not yet aware of the emergency braking. This can be seen in the upper part of figure 3.1. The quick response of the CF-vehicles results in safer headways (shown in the lower part of the figure) that is gradually overpassed. This visual inspection of this well-controlled simulation does not show the entire functioning of the CF-controller. To find out more about the properties of the CF-controller the platoon simulation was repeated several times and analysed in more detail because it is an instructive but not too complex case. During this analysis all state variables were tracked and the effect of different parameter settings of the CF-controller as well as slight modifications to the CF controller logic could be analysed. 3.2 CF simulation results The expected result of the CF simulation is represented in figure 3.2. This picture was obtained during the specification phase and represents the accelerations of 6 cars driving in a platoon. The CF controller is similar to the specifications given in this report. However the communication between cars and the management of messages is simplif ied: the first vehicle (auto 0; performing a predefined acceleration pattern) transmits its own speed continually which is received by vehicle, which is the

28 14 99/NK/162 only CF vehicle in the platoon reacting continuously to the warnings. The figure shows the potential of the CF concept for damping out emerging shock waves: the deceleration is reduced with 0%. Figure 3.2: Evolution of acceleration in time in a simplified platoon experiment with the platoon leader (auto 0) sending its speed continually. Only vehicle has a CF controller that responds to the continuous warning signal (no management of messages necessary in this case) The detailed analysis of the platoon simulation with full functionality (management of messages, threshold for sending warnings) shows a different behaviour and reveals some interesting properties of the current specification of the CF-system. This experiment allows to identify the most important issues without which the design of a well-functioning CF-system is impossible. These issues can be summarised as follows:?? The nature of the CF-controller: the initial response of a damper is excessive because initially the speed difference between the own speed and the speed warning is large.?? The management of warnings coming from different times or different vehicles leads to discontinuous variations of the accelerations.?? The discontinuous switching between different controller modes: free flowing, car-following (ACC functionality) and Co-operative Following; this leads to discontinuous variations of the accelerations. These issues are discussed in the next paragraphs.

29 99/NK/162 1 Nature of the CF-controller Figure 3.3 shows a typical result of a simulation with the current CF specification. The graph shows the acceleration against time of cars in a platoon as in figure 3.1. Autonomous behaviour (here ACC logic is assumed) is compared to Co-operative behaviour. The CF vehicle reacts immediately indeed so that a safe headway is preserved during the braking manoeuvre (see also figure 3.1). However, the response to the speed warning is excessive. Therefore the receiving vehicle exceeds the threshold for sending a warning again and the disturbance is passed to CF vehicles wider upstream. In this way the shock wave will not damp out and can even be amplified CF-Veh acc AICC-Veh 3 acc AICC-Veh 2 acc AICC-Veh 1 acc Platoon leader acc Figure 3.3: Evolution of acceleration in time during the platoon experiment. Although for simplicity reasons the time delays of the ACC- and CF-controllers have not been modelled in this Excel simulation the fast but excessive initial reaction of the CF controller can be observed. The cause of this behaviour is the combination of two characteristics of the CF-controller:?? The speed warning can contain either the sender s own speed or the speed of its predecessor. In this way severe speed warnings can be sent (large speed difference with respect to the receiving vehicle).?? The implementation of the CF-controller as a damper that responds proportionally to the speed difference between the own speed and the transmitted speed warning. This causes the first reaction to be maximal (acceleration of -.0 m/s^2) followed by a quick moderation of this reaction in this example even resulting in positive accelerations again.