Preliminary Concepts and Application Areas
1.5 Model Applications and Transportation Systems Engineering
1.5.1 Transportation Systems Design and the Decision-Making ProcessProcess
20 1 Modeling Transportation Systems: Preliminary Concepts and Application Areas
results in a sequence of decisions (plans or projects) taken at different, not neces- sarily predefined, moments in time, with each decision accounting for the effects of previous decisions and exogenous factors. In this framework, the role of quantita- tive methods for the definition and evaluation of alternative projects is even more relevant as they ensure a sort of “dynamic rationality” for the whole process.
The decision-making process described above is often considered a gross simpli- fication of actual public decision-making processes in the real world. Despite this criticism, it should be seen as a reference paradigm that, with necessary adapta- tions, can in principle be applied to very different problems and decision contexts.
The theoretical analyses that have led to “planning theory” as a theory of collec- tive decision-making are beyond the scope of this book. However, identification of the role and limits of transportation systems analysis and design within the broader decision-making process is extremely relevant. To this end, it is useful to consider the main activities of the decision-making process as shown in Fig.1.9. The right- hand side of the figure shows schematically the decision process, and the left-hand side shows the phases of analysis and modeling that support its activities.
In the objectives and constraints identification phase, the objectives of the decision-maker (or decision-makers) and the relevant constraints for the project are defined. Objectives and constraints may be either explicit or, at least partly, implicit.
They depend on the perspective of the decision-maker and, in one way or another, define the type of actions that can be included in the project (e.g., creation of new facilities over the long term or reorganization of existing facilities in the short term).
Modifications to the transportation system can be designed and evaluated from different points of view. Objectives of a private operator, for example, would typ- ically include profit maximization. Constraints might include existing regulations, the available budget, service or fare obligations, the technical limits on the pro- duction capacity of the factors employed, and so on. In the case of public decision- makers, the project objectives are numerous, often not clearly defined and frequently conflicting with each other, as, indeed, are the interests of a “complex” society.
A public decision-maker may be interested in increasing safety, reducing the gen- eralized transportation cost borne by the users, increasing equity in the distribution of transportation benefits, improving accessibility to economic and social activities, fostering new land development, protecting environmental resources, and reducing the public deficit. Objectives and constraints, explicit or implicit, synthesize the val- ues and attitudes of the firm or of society.
The increasing importance of energy consumption and environmental conserva- tion in recent decades is a clear example of this point. Both objectives and con- straints influence the successive phases of the process, especially the analysis of the present situation and the actions that can be included in alternative projects.
From the modeling perspective, these factors have an impact on the definition of the analysis system, that is, identification of the elements and their relationships, which are included in the representation of the system in order to evaluate correctly the effects of planned actions. In theanalysis of the present situationphase, data on the transportation and activity systems are collected. Data are used to analyze the present system state and identify its main deficiencies or “critical points” with
22 1 Modeling Transportation Systems: Preliminary Concepts and Application Areas
Fig. 1.9 Transportation systems design and the planning process
respect to the project objectives and constraints. These critical aspects should be corrected or mitigated by the planned actions. This phase is also linked to thebuild- ing of a mathematical model of the present system, because it provides the input data for the models (supply, demand, land use). Furthermore, the models often pro- vide some system performance indicators (e.g., flows, saturation levels, generalized transportation costs by the O-D pair) that would be impossible or too costly to mea- sure directly.
The next step is theformulation of system projects (or plans), that is, sets of com- plementary and/or integrated actions that are internally consistent and technically feasible.4The strict interdependence among the elements of a transportation system generally requires that a project be designed taking into account the other system
4Complementary projects have mutually reinforcing positive effects (e.g., building park-and-ride facilities and improving railway services), whereas integrated projects aim at reducing possible negative interactions (e.g., upgrading public transportation and increasing parking prices).
components that may be significantly influenced by it. A new subway line, for ex- ample, requires a reorganization of the surface transit lines to increase the catchment area of the stations (complementary action). Restricting the access of cars to parts of an urban area requires the design of appropriate parking areas, transit lines, pricing policies, and so on in order to alleviate its potentially negative effects (integrated actions). System design is usually limited to the definition of the functional char- acteristics of the elements composing the system; their physical design, if required, pertains to other branches of engineering.
In general, several alternative projects can be proposed in response to predefined objectives. One alternative is the nonintervention (do nothing) option. More realis- tically, thedo minimumoption involves implementingcommitteddecisions (those that, for political or other reasons, cannot be reversed) as well as carrying out basic activities required to keep the system state from deteriorating unacceptably. When a complex project involves multiple actions that cannot be implemented simultane- ously, alternative time sequences can be generated, with each sequence considered as an alternative project. Indeed, the impacts of such projects may be significantly influenced by the specific sequence of actions undertaken for their implementation.
Assessment and evaluation of alternative projects require theprediction of the relevant impactsof their implementation. Most of the impacts can be forecast quan- titatively using the mathematical models and their application methods that are de- scribed later in this book. If evaluation of a project requires prediction of its main impacts over a sufficiently long time horizon, assumptions are needed regarding the anticipated future structure of the activity system, or rather the values of the variables that are exogenous to the model. A set of consistent assumptions on the activity system is usually known as asocioeconomic scenario. The evolution of ex- ogenous variables over long time periods depends on complex phenomena related to the demographic, social, and economic evolution of the area and on the related external environment. It is very difficult, and perhaps impossible, to forecast these phenomena with precision. Thus, the usual practice is to consider a number of dif- ferent future scenarios to assess the range of variation of the predicted impacts, and to check the robustness of the alternative projects with respect to the different sce- narios.
Technical assessment of the projects concludes the system design phase. This activity verifies that the elements of the supply system will function within their ranges of economic validity and technical feasibility (e.g., that the forecast user flows are not too low or too high with respect to their technical capacity). Moreover, the technical feasibility of the assumed performance of system components and the consistency of this performance with the forecast system state are ascertained. Tech- nical assessment is based on predicted project impacts. Modeling studies can (and often do) influence the high-level design of projects as, indeed, is usually the case in engineering systems design.5
5This assumes that potential projects are exogenously specified prior to analysis; this is the ap- proach most commonly used in applications. However, mathematical models can also be used as supply design tools, as discussed in Chap. 9. As stressed in that chapter, supply design models
24 1 Modeling Transportation Systems: Preliminary Concepts and Application Areas Activities related to the analysis of the present situation, formulation of alterna- tive projects, prediction of relevant impacts, and technical assessment can together be defined as thesystem design phase.
The predicted impacts of alternative projects can be further processed to facili- tate their comparison. There are many techniques for the analysis and comparison of alternative projects with different levels of aggregation. However, it should be stressed that these techniques cannot and should not replace the actual decision- making process, which is based on compromises among conflicting interests and objectives. Rather, they should be considered as tools to support decision-making.
After a project, or part of it, is implemented, one can compare forecast and ac- tual effects, note the occurrence of unexpected developments and new problems, and evaluate social consent or dissent. These observations may modify some elements of the project or alter its future development. Projectmonitoring6is the system- atic checking of the main “state variables” of the transportation system using these checks for the a posteriori evaluation of project impacts and the identification of new problems. Monitoring can also identify deficiencies in modeling and analysis, and suggest areas needing improvement. In practice, monitoring transportation systems and projects is often neglected or carried out nonsystematically, although it should play a much more important role in the planning process.
The complexity of the decision-making processes for transportation systems is clear from what has been said so far. The analyst has a technical role in the phases of analysis, design, and forecasting. It should also be recognized that in general the transportation systems engineer does not have all the technical skills required for all the tasks involved. Interaction with specialists from other disciplines (other branches of engineering, economics, urban and regional planning, and social sci- ences) is needed, particularly if the projects are likely to have significant effects on external systems. On the other hand, understanding the “inner working” of trans- portation systems, and therefore their design and quantitative modeling, lies at the core of the professional competence of transportation systems engineers.