ADAPTIVE CRUISE CONTROL -WHAT CAN THE NETWORK OPERATOR EXPECT?
GTeg Marsden Mike McDonald Mark Brackstone
Transportation Research Group, University of Southampton
ABSTRACT
Mercedes launched the first European Adaptive Cruise Control (ACC) system in the new S-Class model at the end of 1998. Since the PROMETHEUS program of research there have been a number of studies in Europe investigating the potential impact of ACC on network efficiency, safety, the environment and driver behaviour. The studies have involved field trials, microscopic traffic simulation and simulator studies. This paper draws together the results from these studies and presents a new approach to examining ACC. The study shows that ACC has limited operating capabilities in peak traffic but will provide substantial environmental improvements outside of these periods.
The paper begins with a state-of-art review of the current understanding of ACC from studies in Europe, USA and Japan. A description of a data collection exercise using an Instrumented Vehicle measuring real driver following behaviour at four sites across Europe is then presented. Microscopic simulation is used to determine the change in following behaviour that would ensue if the following vehicle had been equipped with an ACC system. A discussion follows on the traffic conditions that will provide the operating boundaries for ACC and the potential for ACC to modify current network capacity. The paper concludes by examining the potential interactions between ACC equipped vehicles and current fixed infrastructure telematics applications such as variable speed limits.
The paper is based on work carried out within the 'Deployment of Inter-Urban ATT Test Scenarios' (DIATS) DGVII funded Fourth Framework project.
1. INTRODUCTION
Congestion is estimated to cost the European Union some 2% of GDP every year; accidents another 1.5% and air pollution and noise another 0.6%. This is equivalent to 250 BECU per year for which road transport is responsible for over 90% (EC, 1995). Clearly the efficiency of the transport systems of the European Union is essential to the competitiveness of Europe. However, the development of an efficient transport network within Europe must consider the integration of the networks of a number of modes of transport to enhance mobility in a sustainable way.
The European Commission action programme "Sustainable Mobility: Perspectives for the Future" sets out the objectives for the European Union to implement "sustainable mobility" within the Common Transport Policy (DGVII, 1997). Key elements of this initiative are tile development of "efficient and environmentally friendly transport systems that are safe and socially acceptable."
Fixed "_infrastructure systems have been developed and implemented to improve the efficiency, safety and operating environment of the road network. Examples of fixed- infrastructure systems include:
• Automatic Incident Detection, where abnormalities in traffic flow are detected and warning signs posted to drivers approaching the area using variable message signs;
• Variable Speed Limits, where speed information is provided to drivers based on ambient flow conditions; and
• Ramp Metering, where vehicles are held on the slip-road and allowed to filter onto the main carriageway in a controlled manner.
A full review of these systems and their potential impacts can be found in DIATS (1999a).
Whilst the value of such systems has been established through a number of monitoring studies (Harbord, 1998, HajSalem et al., 1991), attention is increasingly taming to improvements to the flow regime that can be made through Advanced Driver Assistance Systems (ADAS) and Automated Vehicle Guidance (AVG).
ADAS systems provide support to the driver by performing certain parts of the driver's task. An automatic gearbox can be considered to be a form of ADAS although the term is associated more with systems such as Adaptive Cruise Control, where the vehicle intelligently adapts its speed to maintain a preset time gap between it and the vehicle in front.
AVG systems (Zwaneveld and Van Arem, 1998) include ADAS systems as a first stage development. However, the AVG concept also embraces vehicle to roadside communication such as Intelligent Speed Control (Hofmalm et al., 1998) and vehicle to vehicle communication (Berg, 1998, Couch et al., 1999).
This paper presents the results of an in-depth analysis of the first major ADAS system to be launched in recent years, Adaptive Cruise Control (ACC). ACC development has been underway since the PROMETHEUS programme of European research in the late 1980s. Manufacturers completed the pre-competitive collaborative research stage of development in 1994 and have been developing market ready products in the intervening 5 years. Mercedes launched their DISTRONIC system at the Paris motorshow in 1998 although it is still not available to buy, Jaguar and BMW are expected to launch their systems in late 1999.
In all ACC systems the control is based upon a sensor (usually a microwave radar) which measures the distance to the preceding vehicle. The system attempts to maintain a desired speed (controlled by the driver) whilst observing a minimum time headway between the vehicles. First generation ACC are fully independent (not
requiring any vehicle to roadside or vehicle to vehicle communication) and assume that the driver has control of the steering of the vehicle at all times. The systems will only operate in high speed motorway driving (above approximately 40 km/hr). ACC is being marketed by manufacturers as a device to improve driver comfort and perceived safety and is targeted at the luxury vehicle market initially.
Section 2 examines the potential impacts of ACC on traffic network efficiency and the environment. Section 3 examines some of the human factors issues that will accompany the introduction o f ADAS systems. Section 4 describes some of the social and legal issues that will influence the uptake of the system. Section 5 draws together the findings from the reviews and summarises the impacts of ACC from the perspective of a network operator.
2. ACC: N E T W O R K EFFICIENCY AND THE ENVIRONMENT
A number of European studies have attempted to determine the impacts of ACC on network efficiency. The main studies are reported in Marsden et al. (1999a) and Zwaneveld and van Arem (1998). These studies have typically looked at ACC operating on a three-lane motorway with a demand profile at or just above capacity to determine whether ACC has a positive impact on capacity. Some fairly approximate assumptions have been made regarding ACC use (e.g. all users set the same headway, if the system can be used it will be used) and varying percentages of ACC vehicles have been introduced to understand how the changes in network efficiency will devolve over time.
Overall, there is a surprisingly good agreement between the independent modelling exercises. The capacity change for the motorway is linked directly to the relationship between vehicle headways in unassisted driving and those set for the ACC systems, which is intuitively reasonable. Headways of greater than 1.2 seconds for the ACC system were shown to cause small increases in average journey times. Headways of below 1.2 seconds give small capacity gains (less than 5% typically).
Marsden et al. (1999b) examined the reasons for the increase in journey times with larger headways in more detail. The slow lane o f the motorway was dominated by HGV (which formed 15% o f the flow in the study) and very little use was made o f ACC so the speeds remained unchanged. The middle lane of the motorway was affected with a slight drop in average speed but those effects were not statistically significant until 70% of vehicles were equipped with a system target headway of 1.5s. This meant that the majority of the delay was being caused by the fast lane. A target headway of 1.5seconds is greater than the mean headway encountered on the fast lane of a motorway. Figure 1 shows a distribution o f headways during ear following sequences at 110 to 120 km/hr measured using an Instrumented Vehicle on the M3 in the UK. Some o f the delay in the fast lane was attributed to vehicles cutting-in in front of the ACC equipped vehicle. The driver was seen to resume control over the system when the deceleration response o f the system is inadequate. This in turn results in mini shockwaves propagating down the outside lane until the gap between vehicles is large enough to adapt to the deceleration.
The impacts of ACC, irrespective of headway were not found to be significant until greater than 20% of the vehicle fleet was equipped. This highlights the relationship between the existing headway distribution, the target headway for the ACC equipped vehicles and capacity. Less than 20% of vehicles equipped, not all of which will be using the system at any one time, does not have a significant impact on the overall headway distribution and therefore capacity. Above this value and the effects become more significant, particularly as the difference between mean headway and ACC target headway grows.
One of the principal benefits that has been shown from ACC has been the improvement in longitudinal control that the system offers over manual driving. Marsden et al. (1999a) report reductions in the standard deviation of acceleration of between 46 and 52% for following events where the ACC system is suitable for use. This implies reductions in fuel consumption and harmful emissions as the engine can operate in a much more controlled manner with less severe variations in combustion between cycles in the engine chamber. It seems likely that this benefit will be achieved primarily outside of the peak hours when the system can be operational for longer periods of time.
3. ACC: H U M A N F A C T O R S ISSUES
There are a number of hurnan factors that will influence the successful widespread introduction of ACC. These include not only the design of the interface but also the response of the system. If the ACC system does not respond to a vehicle ahead in a way that is acceptable to the driver then the system will not be used. In addition to the concerns regarding the system interface and performance, there are also concerns about the long term effects of driver assistance devices. Will drivers lose some skill as a result of handing over control to an automatic system and if so what are the safety implications of this?
A list of issues identified within DIATS (DIATS, 1999b) relevant to the human acceptance of ACC are:
l. Does the user trust in the safety? 2. When will drivers use the system? 3. How much will drivers use the system? 4. What characteristics should the system have? 5. What are the HMI issues?
6. What effect do demographics have?
7. What effects do national driving characteristics have? 8. What are the long term effects of using the system?
This section provides a brief review of evidence that is available on these points. Further evidence is discussed in Brookhuis and de Waard (1999).
The US Field Operational Trial (Fancher et al., 1997) has provided the largest body of results on general user acceptance issues. In the study, over 100 people were given an A C C vehicle to drive for either two or five weeks. The overwhelming opinion of the drivers is that they liked the system and trusted the system. Drivers tried to use the
system as much as they could which was about half of all miles travelled above 35mph and for 39% of the time it was available (i.e. the vehicle was travelling above the miuimurn ACC operational speed threshold). The system used for the Field Trial only allowed deceleration to 0.09g, which does not include active braking (first generation commercial systems are likely to allow deceleration to 0.2 or 0.3g). This meant that the system was unsuitable for operation near junctions and in busy periods where vehicles cut-in in front of the ACC system and the response of the system was not suitable for the driver. The ACC system only has knowledge of the vehicle directly in front of it whereas a driver has a view of all lanes and an anticipation of the position ahead. It is perhaps not surprising therefore that the drivers should choose to take control of their vehicles during such busy conditions where their sensing skills are necessary.
One result of the trial, supported by work by McLaughiin and Serafin (1999), is that drivers, when faced with a situation where they have to resume control over the ACC system, as the braking provided is insufficient, begin braking later and brake harder. This is a potential safety concern, particularly when ACC vehicles are operating with non equipped vehicles that will have to react to the stronger braking using manual reactions. At this early stage in ACC development, it may be that driver's had a incorrect perception of the system performance and that this would be learnt over a period of time. There is therefore, a strong case to be made for a behaviour monitoring study to be performed in Europe to establish such effects and also to monitor the long term driver behaviour and skill changes as these systems begin to penetrate the market.
Other studies into the safety impacts of ACC devices (Touran et al. (1998) and Vaa and Vatn (1998)) have examined the potential safety benefits of ACC that will result from the driver receiving a cue from the system that deceleration is required much earlier than a driver would be able to perceive through a visual stimulus. Safety concerns have been central to the development of ACC systems and have perhaps delayed the launch of the system. Bad publicity of the system could set back development and market penetration by several years so a cautious approach is being adopted.
Vehicle manufacturers have spent a considerable effort in determining the correct interface and range of user selectable functions for ACC (e.g. Kopf and Nirschl, 1997). No official ISO standard for ACC systems has yet been approved. However, work is ongoing to ensure that a standard minimum set of rules regarding the information that will be provided to the driver is provided. This will include:
• an indication o f if the system is operating;
• whether the system has acquired and is following a target vehicle; • what the cruise speed of the vehicle has been set at;
• what the target headway has been set at.
A stated preference questionnaire, examining the willingness o f drivers to purchase and use an ACC system performed in Belgium, Germany and Norway (DIATS, 1999a) identified the following groups o f people as more likely to use the ACC system:
• People describing the way they drive as careful (Belgium, Germany and Norway) • People who have an automatic gearbox on their current car (Belgium, Germany
and Norway)
• People with children under 15 in their household (Belgium and Norway) • People aged 50 or above.
Although the stated preference questionnaire is only asking the opinion of drivers who have not directly experienced the system, it seems there may be a greater market share for careful, older drivers. This evidence, combined with the simulation results which show that little network impact will be seen until more than 20% of vehicles are equipped, indicates that it will be a number of years before ACC modifies driver behaviour and flow characteristics significantly enough to make a network impact. 4. SOCIAL AND LEGAL ASPECTS OF ACC
The stated preference survey conducted within DIATS (DIATS, 1999a, 1998a) is the widest user preference questiormaire survey conducted on ACC within the EU to date. The questionnaires examined user willingness to buy ACC, Collision Warning and Stop and Go systems (for low speed longitudinal control) and also willingness to use the system if it were fitted as standard on their vehicle. The study highlighted several interesting findings in addition to those reported in Section 3. A difference in price between the three countries where the surveys were conducted which would achieve a 50% market share was found:
* Belgium- 1090 Euro * Norway - 2125 Euro • Germany - 235 Euro
Some of the difference in price will be due to differences in standards of living. Norway has traditionally shown a higher standard of living with a greater propensity to purchase new technology, with the mobile phone being a precedent to this. What is interesting is the low price from the German sample. The Collision warning system (which provides a warning, but no assistance, to the driver if approaching a vehicle ahead in a dangerous manner) was preferred to the ACC system in Germany. This is perhaps indicative of the different driving styles on the autobahns with unrestricted speed limits and those in Norway (60mph).
The overwhelming conclusion from the survey in Belgium was that 94% of people would use ACC and that the system fulfils a perceived need from the driver. This supports evidence from the US Field Trial where drivers reported positive feedback in all cases and a desire for the system to do more for them. It seems inevitable therefore that such systems will become integrated into the vehicle fleet. Perhaps the largest single factor that has delayed the introduction of these systems onto the market has been the legal concerns following an accident which results from automating this element of the driver task should
A study of the legal implications o f ACC was undertaken as part of the DIATS study (1998b). Further work is currently on-going in the RESPONSE European project which is examining issues relating to the type approval of new driver assistance
systems and the issues regarding testing the systems to an acceptable level for Governmental approval. The DIATS study highlighted legal issues in four areas. Currently recommended headways are somewhat greater than those observed on busy motorways (e.g. 2 second rule for UK Highway Code). In the event of accident involving a vehicle fitted with ACC, the liability, based on negligence or fault may be conside_rably affected by three factors:
1 If the target headway has been adjusted by the driver, which will be possible with the currently proposed ACC systems, to a value below the nationally recommended minimum the driver may be considered to have failed to abide by the standards required of a reasonable driver.
2 Actions for damages brought against the supplier or manufacturer of ACC equipment may make use o f similar arguments. The defence that the driver selected a headway below the recommended limit may be employed should such a situation arise. However, the design of a system capable of a headway below the recommended limit could be interpreted as a product defect.
3 It is theoretically possible that criminal proceedings may be commenced against the driver of an ACC vehicle for careless driving if the headway is set below the national limit.
It is clear that drivers currently drive below the nationally recommended headway limit and that it is not inherently unsafe to do so. However, the burden of proof of negligence on a driver with an A c e system set below this limit is considerably easier. IfACC drivers set their headways at the national limit, this will have an adverse effect on network capacity as explained in section 2. It is therefore recommended that the process of altering officially recommended headways be examined.
Current ACC systems are designed for use on motorways. Systems that adequately deal with low speeds, sharp bends and crossing traffic are not likely for some time. It may therefore be appropriate to seek legal provisions to discourage use of ACC on inappropriate roads.
As discussed in Section 3, a draft standard for ACC system design is under development but, like many standards, the progress is slow. An international standard that addresses system design and use is desirable.
It may be possible for the safe operation of the components o f an ACC system to be assessed during the annual vehicle maintenance check. However, as is the case with Anti-lock Braking Systems (ABS), the legal responsibility for ensuring that the ACC system stays in efficient and safe working order will lie with the operator of the vehicle.
5. CONCLUSIONS
The impacts of Adaptive Cruise Control on the efficiency of the road network are likely to be small in the next 10 years. Indications that market penetration levels of at least 20% will be required before any change to efficiency will be realised. It is important to accept that ACC is a comfort based driving aid and that the system was
not designed to improve road capacity. However, the potential for ACC to moderate capacity has been identified. If drivers select, and are allowed to select, headways of 1.2 seconds or less, then small increases in capacity will be possible. If however, users select headways above those typically observed on motorways today of about 1.2 to 1.4 seconds, then a negative impact on capacity may occur.
The mai_'n benefit of ACC appears to be for the individual driver in performing the longitudinal control task much better than the average driver is able to. Significant reductions in acceleration variation during car-following processes have been identified. This will reduce fuel consumption and harmful exhaust emissions.
The safety of the systems has been carefully considered in the design of the systems and this has contributed to the delay of the widespread system introduction with fears of bad publicity and liability claims. Some studies have estimated safety benefits whilst others recommend a monitoring of driver skill levels which may depreciate with automation of the driving task.
Considerable research has been conducted into the legal implications of ACC. The largest issue with the introductions of ACC is the need to set the system headway below current nationally recommended safe limits (of around 2 seconds) to avoid losing capacity and to avoid vehicles cutting in, in front of an ACC user. The potential liability of the driver and manufacturer in the result of an accident whilst using a system set below the nationally recommended following headway is a concern.
Finally, it should be noted that there is a general acceptance of the desirability of an ACC system or some other driver assistance devices (such as Collision Warning) amongst the driving population. Evidence from a US field trial had very positive feedback regarding the system. ACC will reach the market shortly and further monitoring during the early stages of deployment is essential to assess the long term impacts of such systems.
6. REFERENCES
Benz, T. (1998) Praxis-Based Multi-Stage Evaluation of Advanced Vehicle Control Systems on the Example of CHAUFFEUR. Proceedings of the 5 th Annual Worm Congress on 1TS, Korea.
Brooldauis, K. and de Waard, D., (1999) The Human Factor in Advanced Driver Assistance Systems. Proceedings of the Advanced Driver Assistsnee Systems: Vehicle Control for the Future Seminar. IMechE, London.
Couch, N., Waterhouse, J., Wu, J., Brackstone, M., McDonald, M. and Lansdown, T. "Technologies for Advanced Cooperative Driving - A Study of the Operation of Road Convoys". Proceedings of the Advanced Driver Assistance Systems: Vehicle Control for the Future Seminar. IMechE, London.
DIATS (1998a) Deliverable 13-14: User Preference and Behavioural Changes from ATT Systems. DGVII Project RO-96-SC.301, CEC, Brussels.
DIATS (1998b) Deliverable 16: Assessment of Legal Issues Relevant to the Deployment of ATT systems. DGVII Project RO-96-SC.301, CEC, Brussels.
DIATS (1999a) Final Report. DGVII Project RO-96-SC.301, CEC, Brussels.
DIATS_(1999b) Deliverable 17: Evaluation of ATT System Scenario Deployment Options. DGVII Project RO-96-SC.301, CEC, Brussels.
DGVII (1997) The Common Transport Policy Sustainable Mobility: Perspectives for the Future. Commission Communication to the Council, European Parliament, Economic Social Committee and Committee of the Regions
EC (1995) COM(95)691 final, 20.12.1995 Towards Fair and Efficient Road Pricing in Transport - options for internalising the external cost of transport in the European Union - Green Paper.
Fancher, P., Ervin, R., Sayer, J., Hagan, M., Bogard, S., Bareket, Z., Mefford, M., Haugan, J. (1997) Intelligent Cruise Control Operational Test - Interim Report, August 1997. DOTHS 808 622. NTIS, Virginia, USA.
Hajsalem, H. & Alii (1991) Isolated Ramp metering Strategies: Real-life study in France project Christiane, DGXIII programme ~< Drive 1 ~>, Report n ° 7A, 713 and 13 Harbord, B. (1998). M25 controlled moturway - results of the first two years. Proceedings o f the 9th International Conference on Road Transport Information and Control. Conference Publication no 454. IEE, London, UK:
Hofmarm, O., Brockmarm, M., Carrea, P. and Watts, D. (1998) Urban Drive Control - An Integrated and user-oriented approach for telematic assistance of longitudinal Vehicle Control. Proeeedings of the 5 th Annual World Congress on ITS, Korea. Kopf, M. and Nirschl, G. (1997) Driver-Vehicle Interaction while Driving with ACC in Borderline Situations. Proeeedings of the 4 th World Congress on ITS, Berlin, Germany.
Marsden, G., McDonald, M. and Bkackstone, M. (1999a) Towards an Understanding of Adaptive Cruise Control. Submitted to Transportation Research C, May.
Marsden, G., McDonald, M. and Brackstone, M. (1999b) Inter-urban scenario assessment of Adaptive Cruise Control. Proeeedlngs o f the Advanced Driver Assistsnce Systems: Vehicle Control for the Future Seminar. IMechE, London.
McLaughlin, S. And Serafin, C. (1999) Measurement of Driver Intervention Responses During Transition from ACC Deceleration to Manual Control. Proceedings ITSAmerica Conference, Washington D.C. USA. ITS America.
assessing the safety impacts of autonomous intelligent cruise control. Proceedings
ASCE '98 Annual Convention, Boston, USA.
Vaa, T. and Vatn, J. (1998) "Safety aspects by using ITS in the transport sector" Proceedings of Intelligent Transport Systemer Conference 26 - 27 November 1998, Oslo.
Zwaneveld, P. and van Arem, B. (1998) Traffic Effects of Automated Vehicle Guidance Systems. Proceedings of the 5 th Annual World Congress on ITS, Korea. 7. ACKNOWLEDGEMENTS
This work was performed as part of the CEC DGVII funded project DIATS (Deployment of Inter-urban ATT Test Scenarios). The authors would like to thank all of those members of the consortium who have discussed the implications of ACC modelling and also Dr. Jianping Wu for his assistance with coding of the modified FLOWSIM model.
