《系统工程》课程教学资源（英文文献）An expert system to advise on urban public transport technologies

ANEXPERTSYSTEMTOADVISEONURBANPUBLICTRANSPORTTECHNOLOGIESABSTRACT. An important feature of the "sustainable city"concept is a requirement to provideattractive public transport alternatives to the car. In order to achieve this, some cities will require anew public transport system. However, there is no systematic method of deciding which publictransport technology is the most appropriate in a given situation. Expertise in this area is limited tothose who have implemented a new public transport system in their own city, and such expertise is notreadily available to those embarking on the process. The aim of the UTOPIA (Urban TransportOperations and Planning using Intelligent Analysis) project is to develop an expert system to assistwith the decision making process in cities investigating the development of a new public transportsystem. This paper outlines the features required of an expert system, then it goes on to discuss theknowledge acquisition techniques used to elicit information about fourteen British and thirty-twonon-British systems which are either planned or became operational after 1976. Analysis of theinformation gives general knowledge about the nature of the cities, the systems, and the detailed rulesused inthedecisionmakingprocess.INTRODUCTIONCar ownership is increasing rapidly, leading to more road congestion, more use of energy andmore environmental damage.There is much concern about theneed tomake cities"sustainable",thatis, to adopt strategies that are not detrimental to future generations. A key feature of such strategies isthe need to provide attractive alternatives to the car. In many cities, public transport has been allowedto degenerate through lack of adequate investment. It is recognized that in some cities there is a needto provide a new public transport system. There are a variety of possible systems: suburban rail, metro,light rail, improved bus services, busways, guided buses and so on, or some combination of these. Arange of such systems exists in cities around the world. However, there is no systematic way ofdeciding which technology is the most appropriate. Hence there is a danger that each decision will bemade from first principles. In fact, many of those involved in such decisions make only one suchdecision during their life. Therefore, there is a need to transfer experience between one city andanother. Oneway is to encapsulate knowledge about the decision-making process in one city, and thentoapply itelsewhere

AN EXPERT SYSTEM TO ADVISE ON URBAN PUBLIC TRANSPORT TECHNOLOGIES ABSTRACT. An important feature of the "sustainable city" concept is a requirement to provide attractive public transport alternatives to the car. In order to achieve this, some cities will require a new public transport system. However, there is no systematic method of deciding which public transport technology is the most appropriate in a given situation. Expertise in this area is limited to those who have implemented a new public transport system in their own city, and such expertise is not readily available to those embarking on the process. The aim of the UTOPIA (Urban Transport Operations and Planning using Intelligent Analysis) project is to develop an expert system to assist with the decision making process in cities investigating the development of a new public transport system. This paper outlines the features required of an expert system, then it goes on to discuss the knowledge acquisition techniques used to elicit information about fourteen British and thirty-two non-British systems which are either planned or became operational after 1976. Analysis of the information gives general knowledge about the nature of the cities, the systems, and the detailed rules used in the decision making process. INTRODUCTION Car ownership is increasing rapidly, leading to more road congestion, more use of energy and more environmental damage. There is much concern about the need to make cities "sustainable", that is, to adopt strategies that are not detrimental to future generations. A key feature of such strategies is the need to provide attractive alternatives to the car. In many cities, public transport has been allowed to degenerate through lack of adequate investment. It is recognized that in some cities there is a need to provide a new public transport system. There are a variety of possible systems: suburban rail, metro, light rail, improved bus services, busways, guided buses and so on, or some combination of these. A range of such systems exists in cities around the world. However, there is no systematic way of deciding which technology is the most appropriate. Hence there is a danger that each decision will be made from first principles. In fact, many of those involved in such decisions make only one such decision during their life. Therefore, there is a need to transfer experience between one city and another. Oneway is to encapsulate knowledge about the decision-making process in one city, and then to apply it elsewhere

A project to do this is being carried out at University College London, in the University ofLondon Centre for Transport Studies. The project, named UTOPIA (Urban Transport Operations andPlanning using Intelligent Analysis), has the following objectives:· to develop an understanding of how decisions are made about the most appropriate publictransport system for an urban area,?to encapsulate that knowledge in an expert system;totransfer thatknowledge elsewhere?In the next section the nature of expert systems and why they are an appropriate methodology arediscussed. Then the approach being used in the project is described. A method of constructing theexpert system is then presented. Finally, further work is identified and conclusions are drawnEXPERTSYSTEMSExpert systems have been defined as "sophisticated computer programs that manipulateknowledge to solve problems efficiently and effectively in a narrow problem area" (Waterman, 1985)Assuch,theyareusedtosolveproblemswhichusuallyrequirehumanexpertiseorknowledge,forexample, planning, diagnosis and interpretation. It is the explicit inclusion of human knowledge in theexpert systemwhich distinguishes it from othertypes ofcomputer program.An expert system will include two types of knowledge:factual knowledge and heuristicknowledge (including "rules of thumb" and problem solving strategies) (Hayes-Roth, Waterman, &Lenat, 1983). Both types of knowledge are required to solve a problem. While factual knowledgeabout the domain can usuallybe obtained from conventional sources (for example,publications),heuristic knowledge must be obtained from a human expert. The process by which this is achieved isknown as "knowledge acquisition" The method of knowledge acquisition used in the UTOPIA projectisdiscussed inthefollowingsection.An expert system solution is not appropriate for all types of problem. Indeed, Waterman (1985)states that expert system development should only be attempted if it is "possible, justified andappropriate".Waterman's rules for determining when these criteria are satisfied are given in Figure 1.It was felt that these rules actually do apply to the domain of urban public transport planning. Inparticular:1.experts exist (public transport systems have been built) and preliminary investigations showed thatthey can articulate their approach to the problem;2. experts are scarce (relatively few systems have been built or authorized) but there are many

A project to do this is being carried out at University College London, in the University of London Centre for Transport Studies. The project, named UTOPIA (Urban Transport Operations and Planning using Intelligent Analysis), has the following objectives:  to develop an understanding of how decisions are made about the most appropriate public transport system for an urban area;  to encapsulate that knowledge in an expert system;  to transfer that knowledge elsewhere. In the next section the nature of expert systems and why they are an appropriate methodology are discussed. Then the approach being used in the project is described. A method of constructing the expert system is then presented. Finally, further work is identified and conclusions are drawn. EXPERT SYSTEMS Expert systems have been defined as "sophisticated computer programs that manipulate knowledge to solve problems efficiently and effectively in a narrow problem area" (Waterman, 1985). As such, they are used to solve problems which usually require human expertise or knowledge, for example, planning, diagnosis and interpretation. It is the explicit inclusion of human knowledge in the expert system which distinguishes it from other types of computer program. An expert system will include two types of knowledge: factual knowledge and heuristic knowledge (including "rules of thumb" and problem solving strategies) (Hayes-Roth, Waterman, & Lenat, 1983). Both types of knowledge are required to solve a problem. While factual knowledge about the domain can usually be obtained from conventional sources (for example, publications), heuristic knowledge must be obtained from a human expert. The process by which this is achieved is known as "knowledge acquisition". The method of knowledge acquisition used in the UTOPIA project is discussed in the following section. An expert system solution is not appropriate for all types of problem. Indeed, Waterman (1985) states that expert system development should only be attempted if it is "possible, justified and appropriate". Waterman's rules for determining when these criteria are satisfied are given in Figure 1. It was felt that these rules actually do apply to the domain of urban public transport planning. In particular: 1. experts exist (public transport systems have been built) and preliminary investigations showed that they can articulate their approach to the problem; 2. experts are scarce (relatively few systems have been built or authorized) but there are many

proposed systems;3. the planning process cannot be accomplished solely by using conventional mathematicalmodels--experts take qualitative criteria into account and apply heuristic rules; andIFtask does notrequire commonsense ANDtask requires only cognitive skills ANDexpertscanarticulstetheirmethodsANDgemuineexperts existANDexperts agree on sohutions ANDtask is not too difficult ANDtask is not poorly understoodTHEN expert system approach possibleIFtask sohution has a high pay-off ORhuman expertise is being lost ORhuman expertise is scarce ORexpertise needed im manylocations ORexpertise needed in hostile environmentTHEN expert system development justifiedIFtaskrequiressymbolmanipulationANDtask requires heuristic sohutions ANDtask is nottoo easy ANDtask has practical vahue ANDtask is ofmanageable sizeTHENexpertsystemdevelopmentappropriateFIGURE 1. Rules used to determine if expert system development is possible, justified andappropriate (from Waterman, 1985)4. the task has practical value, in terms of improved public transport, and has a high payoff (thedevelopment of a new public transport system is expensive, but an appropriate system will benefit acitysignificantly)However, it is not possible to establish if the development of an expert system is actuallyfeasible untila complete understanding of the decision-making process has been obtained from the knowledgeacquisition phase of the project.If expert system development is appropriate for a particular problem, the resulting system wouldnormallydisplaythefollowingfeatures.· Auser interface. A typical expert system will obtain information from the user, via a question-answerdialogue, about the current problem or scenario. Expert systems have been (somewhat ambitiously)likened to"an expert on the end of the 'phone".In some cases the user will be allowed to volunteer

proposed systems; 3. the planning process cannot be accomplished solely by using conventional mathematical models-experts take qualitative criteria into account and apply heuristic rules; and FIGURE 1. Rules used to determine if expert system development is possible, justified and appropriate (from Waterman, 1985). 4. the task has practical value, in terms of improved public transport, and has a high payoff (the development of a new public transport system is expensive, but an appropriate system will benefit a city significantly). However, it is not possible to establish if the development of an expert system is actually feasible until a complete understanding of the decision-making process has been obtained from the knowledge acquisition phase of the project. If expert system development is appropriate for a particular problem, the resulting system would normally display the following features. • A user interface. A typical expert system will obtain information from the user, via a question-answer dialogue, about the current problem or scenario. Expert systems have been (somewhat ambitiously) likened to "an expert on the end of the 'phone". In some cases the user will be allowed to volunteer

information about the problem before the system requests information. Clearly the user's answers willhave a restricted format (such as yes/no answers, numerical values or selection from a menu)Typically, the users can indicate the degree of confidence they have in an answer.·Explanation.One of the hallmarks of human experts is their ability to justify the conclusion reached,or to explain why a particular item of information is needed. Therefore, for an expert system to becredible, it must be able to give plausible explanations for its decisions. In addition, during the initialinformation-gathering dialogue, the expert system should be able to explain why a particular item ofinformation is requested.?Graceful degradation. An expert system should show graceful degradation, that is, if the informationgiven is uncertain, incomplete or contradicts the knowledge it has about the domain, the expert systemshould produce a less confident answer rather than none at all. The expert system should also be ableto identify when a problem lies outside its sphere of expertise.From the system developer's point of view, an expert system consists of four main components..The knowledge base contains both factual, and heuristic, domain-specific knowledge. Suchknowledge is usually represented using rules, but frames may be used to represent hierarchicalknowledge. Rules are generally of the form:IF Condition AND Condition... THEN ConclusionA conclusion can be deduced if all the rule conditions are satisfied. (FigureI consists of rules definingwhen expert system development is appropriate)..The working memory or global database contains the factual knowledge supplied by the user aboutthe current problem, along with any deductions which have been made based on that knowledge..The inference engine uses the information in the knowledge base and the working memory to solvethe problem. It has two roles: first, selecting which rules to investigate (the strategy), and second,testing if the selected rule is satisfied, and what conclusion can be drawn. If a rule cannot be satisfiedexactly, or some of the rule conditions are uncertain or unknown then rather than the system failing, atentative or uncertain conclusion should be drawn..The explanation system, which was discussed above.One important, but regrettable, feature of expert systems is that they make mistakes. Expert systemsaddress problems for which a mathematical or algorithmic approach is not appropriate. An expertsystem relies on human expertise and since experts are fallible, expert systems will also be. The mostimportant component of an expert system is its knowledge base. The quality of the knowledge within

information about the problem before the system requests information. Clearly the user's answers will have a restricted format (such as yes/no answers, numerical values or selection from a menu). Typically, the users can indicate the degree of confidence they have in an answer. • Explanation. One of the hallmarks of human experts is their ability to justify the conclusion reached, or to explain why a particular item of information is needed. Therefore, for an expert system to be credible, it must be able to give plausible explanations for its decisions. In addition, during the initial information-gathering dialogue, the expert system should be able to explain why a particular item of information is requested. •Graceful degradation. An expert system should show graceful degradation, that is, if the information given is uncertain, incomplete or contradicts the knowledge it has about the domain, the expert system should produce a less confident answer rather than none at all. The expert system should also be able to identify when a problem lies outside its sphere of expertise. From the system developer's point of view, an expert system consists of four main components. •The knowledge base contains both factual, and heuristic, domain-specific knowledge. Such knowledge is usually represented using rules, but frames may be used to represent hierarchical knowledge. Rules are generally of the form: IF Condition AND Condition. THEN Conclusion A conclusion can be deduced if all the rule conditions are satisfied. (Figure 1 consists of rules defining when expert system development is appropriate). •The working memory or global database contains the factual knowledge supplied by the user about the current problem, along with any deductions which have been made based on that knowledge. •The inference engine uses the information in the knowledge base and the working memory to solve the problem. It has two roles: first, selecting which rules to investigate (the strategy), and second, testing if the selected rule is satisfied, and what conclusion can be drawn. If a rule cannot be satisfied exactly, or some of the rule conditions are uncertain or unknown then rather than the system failing, a tentative or uncertain conclusion should be drawn. •The explanation system, which was discussed above. One important, but regrettable, feature of expert systems is that they make mistakes. Expert systems address problems for which a mathematical or algorithmic approach is not appropriate. An expert system relies on human expertise and since experts are fallible, expert systems will also be. The most important component of an expert system is its knowledge base. The quality of the knowledge within

the knowledge base will largely determine the reliability and the robustness of the expert system. Thenext section looks at how the knowledge required for the UTOPIA expert system was obtainedTHEAPPROACHNew public transport systems in both Britain and abroad are being examined. For the systems inBritaintheapproachusuallyadoptedisasfollows:1. Draw up a questionnaire defining areas of interest;2. Identify suitable experts;3. With each expert:Arrange an interview?Send the questionnaire· Interview the expert using the questionnaire as a basis for discussion (the interview is tape recorded).Transcribe the tape recording of the interview, supplementing it with contemporaneous written notes·Analyse the information toderive strategicknowledge and rules;4. Incorporate the knowledge into the expert system;5. Validate the expert system.If an interview cannot take place (usually the case for systems outside Britain) a written version of thequestionnaire is sent.As this tends toreceive a briefer response than a verbal interview, and since notranscription is necessary, it is much quicker to analyse.A number of topics are included in the questionnaire:1. Objectives of building the system,2. The alternatives considered, both in terms of other modes and other ways of achieving theobjectives, such as road building,3. The factors taken into account when deciding on the most appropriate mode;4. Location:.On the surface but away from the highway· Along the edge or median of the highway, segregated from the highway, except at junctions. On the highway, competing with cars for road space· Use of tunnels or elevated structures;5. The nature of the traffic priority re,me if there are conflicts with road traffic at junctions,6. The effects of utilities, that is gas, electricity, water and telecommunications on the constructionprogramme;

the knowledge base will largely determine the reliability and the robustness of the expert system. The next section looks at how the knowledge required for the UTOPIA expert system was obtained. THE APPROACH New public transport systems in both Britain and abroad are being examined. For the systems in Britain the approach usually adopted is as follows: 1. Draw up a questionnaire defining areas of interest; 2. Identify suitable experts; 3. With each expert: Arrange an interview •Send the questionnaire • Interview the expert using the questionnaire as a basis for discussion (the interview is tape recorded) •Transcribe the tape recording of the interview, supplementing it with contemporaneous written notes •Analyse the information to derive strategic knowledge and rules; 4. Incorporate the knowledge into the expert system; 5. Validate the expert system. If an interview cannot take place (usually the case for systems outside Britain) a written version of the questionnaire is sent. As this tends to receive a briefer response than a verbal interview, and since no transcription is necessary, it is much quicker to analyse. A number of topics are included in the questionnaire: 1. Objectives of building the system; 2. The alternatives considered, both in terms of other modes and other ways of achieving the objectives, such as road building; 3. The factors taken into account when deciding on the most appropriate mode; 4. Location: •On the surface but away from the highway • Along the edge or median of the highway, segregated from the highway, except at junctions • On the highway, competing with cars for road space • Use of tunnels or elevated structures; 5. The nature of the traffic priority re,me if there are conflicts with road traffic at junctions; 6. The effects of utilities, that is gas, electricity, water and telecommunications on the construction programme;

7.Theeffects offunding availability,8. Implications of building the system in terms of the impacts that it has had, and what would havehappened had it notbeen built.8. Implications of building the system in terms of the impacts that it has had, and what would havehappened had it notbeen built.The systems examined in Britain to date are shown in Table 1. A total of fourteen British systemshave been examined.Five are in operation.Only one of these operational systems, the kerb-guidedbus(KGB)system inLeeds,is notlight rail.Two other KGB systemsarebeing considered:inEdinburgh and Hull, while a non-guided busway is being considered for Cleveland. Guided lighttransit (GLT) is being considered in Bristol. (GLT is a hybrid system that uses rubber-tyred articulatedvehicles that can run either in guided mode on a single guide rail with an overhead electric power pickup or can run in unguided mode on the road using electric power from an on-board diesel generator(Freeman & TazeweU, 1993). The operational light rail system in London Docklands is automatic,which has implications in terms of cost and segregation from other vehicles. The proposed ultra lightRail system in the same area is designed to link a proposed residential development area to both theautomatic system and the Jubilee Line Extension of the London Underground, which is currentlyunder construction. A total of thirty-two systems outside Britain have been examined. Twenty areoperational, ten are under development and two proposals have been abandoned (the Honolulu andBrisbane light rail schemes). Of the schemes, six are metros, of which one (the Toulouse VAL) isautomatic, nineteen are light rail schemes, of which five are automatic, and seven are bus-based, ofwhich three use kerb-guidance. Of the systems outside Britain, seven are in other parts of Europe,fourteen in theUnited States,four inCanada,fiveinAustralia,and two inAsia.For fourteen of the systems, eleven of them in Britain, interviews were held, based on asemi-structured questionnaire. In the other cases, the questionnaire was sent by post or e-mail.Table1.Public Transport Systems in Britain Examined

7. The effects of funding availability; 8. Implications of building the system in terms of the impacts that it has had, and what would have happened had it not been built. 8. Implications of building the system in terms of the impacts that it has had, and what would have happened had it not been built. The systems examined in Britain to date are shown in Table 1. A total of fourteen British systems have been examined. Five are in operation. Only one of these operational systems, the kerb-guided bus (KGB) system in Leeds, is not light rail. Two other KGB systems arebeing considered: in Edinburgh and Hull, while a non-guided busway is being considered for Cleveland. Guided light transit (GLT) is being considered in Bristol. (GLT is a hybrid system that uses rubber-tyred articulated vehicles that can run either in guided mode on a single guide rail with an overhead electric power pick up or can run in unguided mode on the road using electric power from an on-board diesel generator (Freeman & TazeweU, 1993). The operational light rail system in London Docklands is automatic, which has implications in terms of cost and segregation from other vehicles. The proposed ultra light Rail system in the same area is designed to link a proposed residential development area to both the automatic system and the Jubilee Line Extension of the London Underground, which is currently under construction. A total of thirty-two systems outside Britain have been examined. Twenty are operational, ten are under development and two proposals have been abandoned (the Honolulu and Brisbane light rail schemes). Of the schemes, six are metros, of which one (the Toulouse VAL) is automatic, nineteen are light rail schemes, of which five are automatic, and seven are bus-based, of which three use kerb-guidance. Of the systems outside Britain, seven are in other parts of Europe, fourteen in the United States, four in Canada, five in Australia, and two in Asia. For fourteen of the systems, eleven of them in Britain, interviews were held, based on a semi-structured questionnaire. In the other cases, the questionnaire was sent by post or e-mail. Table 1. Public Transport Systems in Britain Examined

Table1.PublicTransportSystems inBritainExaminedCitySystemStatusExpertBirminghamLight railAuthorizedRay HughesDavid KeayRobert TarrBristolPlannedLight rail or guided light transitRogerNewportClevelandBuswayPlannedRuth SandersCroydonLight railAuthorizedJon WillisEdinburghKerb-guided busPlannedIanOgdenHullPlannedR.M. FellKerb-guidedbusLeedsLight railAuthorizedAdrianPopeLeedsGuided busOpened1995Adrian PopeLondonDocklandsAutomaticlightrailOpened1987JonWillisUltra light railPlannedBobMillerLondonDocklandsManchesterLight railOpened1992Bill TysonTony YoungLight railNewcastleuponTyneOpened1980Brian MartinTonyRidleyNottinghamLight railPlannedRafael CuestaOpened1994SheffieldLight railDavidAndrewsJohn JordanANALYSISMuch useful information hasbeen obtainedfromthe interviews and postal questionnaires(Mackett & Edwards, 1994). By way of illustration, the information from the British light railschemeswill beconsidered.AsTable2shows,thesystemshavebeenproposedtoachieveavarietyofobjectives.Reduction of traffic congestion is important, as isprovidinga better public transportservice, particularly for those without access to a car, and improving the service to the city centre. Thelatter is related toanother significantobjective,which is to stimulatedevelopment.The new systemswill all tend to improvethe environment,which was mentioned explicitly in onlyone case.Anumberofalternativemodeswereconsidered,asshowninTable3.Improvementstobuses.including busways and the use of KGB, were considered in all cases. Such systems were not thoughtto be adequate because light rail (or GLT) has been adopted or proposed, in all these cases. Briway is aproprietary system using small, automatic guided vehicles on elevated structures. In three cases,Croydon, Manchester and Newcastle, light rail was proposed as a way of upgrading heavy rail lines inneedof renewalandprovidingdirectlinksintothecitycentre.The interview transcripts are examined at two levels: firstly with an aim of abstracting"highlevel" information or factors about the nature of the city and the proposed system as this gives insightinto the required expert system strategy, and secondly, it is necessary to examine the details ofindividual decisions as these give rise to the rules used in selecting the technology. Rules which wereidentifiedfrom more than one interview are of particular interest, although,not surprisingly some of

ANALYSIS Much useful information has been obtained from the interviews and postal questionnaires (Mackett & Edwards, 1994). By way of illustration, the information from the British light rail schemes will be considered. As Table 2 shows, the systems have been proposed to achieve a variety of objectives. Reduction of traffic congestion is important, as is providing a better public transport service, particularly for those without access to a car, and improving the service to the city centre. The latter is related to another significant objective, which is to stimulate development. The new systems will all tend to improve the environment, which was mentioned explicitly in only one case. A number of alternative modes were considered, as shown in Table 3. Improvements to buses, including busways and the use of KGB, were considered in all cases. Such systems were not thought to be adequate because light rail (or GLT) has been adopted or proposed, in all these cases. Briway is a proprietary system using small, automatic guided vehicles on elevated structures. In three cases, Croydon, Manchester and Newcastle, light rail was proposed as a way of upgrading heavy rail lines in need of renewal and providing direct links into the city centre. The interview transcripts are examined at two levels: firstly with an aim of abstracting "high level" information or factors about the nature of the city and the proposed system as this gives insight into the required expert system strategy; and secondly, it is necessary to examine the details of individual decisions as these give rise to the rules used in selecting the technology. Rules which were identified from more than one interview are of particular interest, although, not surprisingly some of

the interviews gave rise to contradictory information. In part this reflects the different problems facedbythecities,forexample,congestion,needforurbanrenewal,andsoonTable 2. Objectives of Constructing New Light Rail Systems in the U.K.CityReduceStimulateBetter serveImprovetheBetter publicOthertrafficdevelopmentthe cityenvironmenttransportcongestioncentreservice1-11Birmingham1III11Bristol-1Croydon11LeedsLondon Docklands!1-LondonDocklands111Manchester1Newcastleupon Tyne1111Nottingham1Sheffield'AutomaticLight RailZUltra Light RailFactors InfluencingTechnologyChoiceFrom the questionnaires and supplementary information in written reports, a number of importantfactors havebeenidentifiedTable 3. Alternative Modes Considered for U.K. Light Rail SchemesCityImproved BuswaysKerb-guidedGuidedTrolleyBriwayUpgradedMetrobusbusbuseslightheavyrailtransit11111Birmingham-1Bristol11CroydonLondon Docklands'111London Docklands?11111111Leeds111Manchester1NewcastleuponTyne1111Nottingham1Sheffield'Automatic Light RailZUltra Light Rail.Nature of the city.The spatial distribution of population and economic activity influences thedemand for travel and hence for the new public transport system. The city may wish to pursue a policyofequity by providing good public transport in areas of low car ownership.Alternatively,the city maybe suffering from serious road congestion and the new system may be intended to reduce this.Constructionofthe system can assist in theprocessofurban regeneration·Nature of the existing public transport system. It may be that the new system is intended to replacean existing system in need of renewal, in which case, the nature of the infrastructure associated with

the interviews gave rise to contradictory information. In part this reflects the different problems faced by the cities, for example, congestion, need for urban renewal, and so on. Table 2. Objectives of Constructing New Light Rail Systems in the U.K. Factors Influencing Technology Choice From the questionnaires and supplementary information in written reports, a number of important factors have been identified. Table 3. Alternative Modes Considered for U.K. Light Rail Schemes • Nature of the city. The spatial distribution of population and economic activity influences the demand for travel and hence for the new public transport system. The city may wish to pursue a policy of equity by providing good public transport in areas of low car ownership. Alternatively, the city may be suffering from serious road congestion and the new system may be intended to reduce this. Construction of the system can assist in the process of urban regeneration. • Nature of the existing public transport system. It may be that the new system is intended to replace an existing system in need of renewal, in which case, the nature of the infrastructure associated with

the latter may be significant. Even if this is not the case, it is important to consider how the newsystem will fit in with the existing systems, which could be heavy rail or buses. In the latter case itmay be possible to re-arrange routes to serve the new system.·Requirements of thenew system.The pattern of demand must be examined interms of bothanticipated patronage and the nature of the factors that influence patronage, for example, travel timesand degree of reliability. These have to be matched to the nature of the systems being considered interms ofcapacity,speed andreliability.·Nature of the proposed routes.The nature of the proposed route pattern can influence the choice oftechnology. Old infrastructure in the form of railway embankments can be useful, since reuse maysavemoneyand reducetheneedfordemolitionof properties.If rails still exist,theymaybereusableby a rail-based form of technology.The desired degree of segregation from the highway can alsoinfluence the decision process. For example, an automatic system must be segregated from all othertraffic, and a guided bus system (either KGB or GLT) can run on ordinary road in the city centre,therebycostingless,butpossiblysufferingfromtheeffectsofcongestion.Generally,thegreaterthedegree of segregation the greater the cost, but the less the influence of other vehicles on travel timesand reliability· Finance.The cost of constructing and operating the system will vary between the varioustechnologies available. In many cases, there is a trade-off between the two. Clearly, the cost must bematched not only to the total funding available, but also to the nature of the funding regime, forexample, the rules under which central government will contribute to the cost.. Political issues. From the evidence based on the British systems, it seems that a local politicalconsensus is essential. Construction of a new system will achieve various objectives, for example,relieving road congestion, improving the environment or aiding urban regeneration. The agenciespromoting the system will have a set of objectives, and the various technologies will contribute tothese indifferent ways.:Importance of proven technology. An innovative technology may offer potential benefits, forexample, automatic vehicles can save on labour costs since there will be no drivers to pay. However, itis much more difficult to assess the costs and practicality of constructing and running such a system.Hence, there may be a case for using proven technology because it reduces the risks involved.. Importance of image. Some modes tend to have a "better image" than others. All other things beingequal, the greater the technological innovation, the better the image and the better the public

the latter may be significant. Even if this is not the case, it is important to consider how the new system will fit in with the existing systems, which could be heavy rail or buses. In the latter case it may be possible to re-arrange routes to serve the new system. • Requirements of the new system. The pattern of demand must be examined in terms of both anticipated patronage and the nature of the factors that influence patronage, for example, travel times and degree of reliability. These have to be matched to the nature of the systems being considered in terms of capacity, speed and reliability. • Nature of the proposed routes. The nature of the proposed route pattern can influence the choice of technology. Old infrastructure in the form of railway embankments can be useful, since reuse may save money and reduce the need for demolition of properties. If rails still exist, they may be reusable by a rail-based form of technology. The desired degree of segregation from the highway can also influence the decision process. For example, an automatic system must be segregated from all other traffic, and a guided bus system (either KGB or GLT) can run on ordinary road in the city centre, thereby costing less, but possibly suffering from the effects of congestion. Generally, the greater the degree of segregation the greater the cost, but the less the influence of other vehicles on travel times and reliability. • Finance. The cost of constructing and operating the system will vary between the various technologies available. In many cases, there is a trade-off between the two. Clearly, the cost must be matched not only to the total funding available, but also to the nature of the funding regime, for example, the rules under which central government will contribute to the cost. • Political issues. From the evidence based on the British systems, it seems that a local political consensus is essential. Construction of a new system will achieve various objectives, for example, relieving road congestion, improving the environment or aiding urban regeneration. The agencies promoting the system will have a set of objectives, and the various technologies will contribute to these in different ways. • Importance of proven technology. An innovative technology may offer potential benefits, for example, automatic vehicles can save on labour costs since there will be no drivers to pay. However, it is much more difficult to assess the costs and practicality of constructing and running such a system. Hence, there may be a case for using proven technology because it reduces the risks involved. • Importance of image. Some modes tend to have a "better image" than others. All other things being equal, the greater the technological innovation, the better the image and the better the public

perception, and therefore the greater the patronage. For example, light rail is generally regarded ashaving a better image than bus-based systems, even if the latter is guided.In the next section the way in which these concepts are translated into rules for use in the expertsystem is discussed.Development of RulesThis section looks in detail at two examples of rule development.First, the decision in bothManchester and Croydon to convert existing operational heavy rail lines to light rail; and second, thedecisions made in Leeds in selecting between light rail and KGB for specific corridors.The rules included in this section use an "intermediate representation" (Gammack, 1992) in that theylie between the expert's informal, and often implicit, statement of a rule and the machine readablerepresentation of the same rule. In particular, the rule conditions are in free-format. The rules use anextended syntax, for example:IF Condition ANDCondition THEN Conclusion BECAUSEExplanationIF Condition OR Condition THEN Conclusion BUT ImplicationThe extended format allows the explanation and implication of a conclusion to be associated witha rule, which will facilitate the provision of meaningful explanations.The first example is the conversion of heavy rail to light rail in Manchester and Croydon, wherethe objective was to link existing city centre railway stations and improve the service offered onselected rail corridors.Inboth cases conversionto busways was rejected,butfordifferent reasons(text surrounded by .. is a quote from the interview transcript).Manchester:"The busway options were eliminated largely on the grounds of capital costs of convertingexisting railway lines -- not even disused ones--to busways. The cost of converting the track to abusway--either guided or unguided--was so high that even taking into account that the buses werecheaper--though more of them would be needed--the total cost of a busway came out to be more thanfor lightrail."Croydon:"The capital costs for the two systems [guided busway and light rail] for civil engineering cameout very similar.Theoperating costs forabusway werehigher becamewe assumed200 placetrams,and the capacity of a single decker bus does not approach this., so the number of vehicles was higherIn terms of total cost the two systems were pretty similar

perception, and therefore the greater the patronage. For example, light rail is generally regarded as having a better image than bus-based systems, even if the latter is guided. In the next section the way in which these concepts are translated into rules for use in the expert system is discussed. Development of Rules This section looks in detail at two examples of rule development. First, the decision in both Manchester and Croydon to convert existing operational heavy rail lines to light rail; and second, the decisions made in Leeds in selecting between light rail and KGB for specific corridors. The rules included in this section use an "intermediate representation" (Gammack, 1992) in that they lie between the expert's informal, and often implicit, statement of a rule and the machine readable representation of the same rule. In particular, the rule conditions are in free-format. The rules use an extended syntax, for example: IF Condition AND Condition THEN Conclusion BECAUSE Explanation IF Condition OR Condition THEN Conclusion BUT Implication The extended format allows the explanation and implication of a conclusion to be associated with a rule, which will facilitate the provision of meaningful explanations. The first example is the conversion of heavy rail to light rail in Manchester and Croydon, where the objective was to link existing city centre railway stations and improve the service offered on selected rail corridors. In both cases conversion to busways was rejected, but for different reasons (text surrounded by . is a quote from the interview transcript). Manchester: "The busway options were eliminated largely on the grounds of capital costs of converting existing railway lines - not even disused ones-to busways. The cost of converting the track to a busway-either guided or unguided-was so high that even taking into account that the buses were cheaper-though more of them would be needed-the total cost of a busway came out to be more than for light rail." Croydon: "The capital costs for the two systems [guided busway and light rail] for civil engineering came out very similar. The operating costs for a busway were higher became we assumed 200 place trams, and the capacity of a single decker bus does not approach this., so the number of vehicles was higher. In terms of total cost the two systems were pretty similar