Preliminary Concepts and Application Areas
1.5 Model Applications and Transportation Systems Engineering
1.5.2 Some Areas of Application
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.
Strategic Planning
Strategic or investment planning involves decisions about long-term (10–20 year) capital investment programs involving the construction of new facilities (e.g., roads, railways, ports) and/or the acquisition of vehicles and technologies (e.g., rolling stock and traffic control systems). In this case, projects usually include transporta- tion services, pricing policies, and, in some cases, travel demand management poli- cies (e.g., access or parking restrictions). Public projects are included in urban, re- gional, national, or transnational transportation plans, depending on the extent of the area; the projects of agencies or companies are part of their strategic development (or business) plans.
For strategic plans, the study generally encompasses the entire transportation sys- tem because substantial changes, even for a single mode, may influence the struc- ture and functioning of the whole system. Returning to the example of an urban transportation plan for a new subway line, the design elements will also include the surface transit lines, parking policy, fare policy, and so on. Evaluation of the line’s effects cannot be limited to the public transportation system because the demand split among modes may well change, producing significant effects on road conges- tion, parking availability, and so on. The time horizon for this level of design re- quires forecasts of alternative activity system scenarios, and the reverse interactions between the transportation system and the activity system need to be considered as well. Continuing with the same example, it is reasonable to expect that construction of a new subway line may affect, to some extent, the pattern of land use and there- fore of travel demand. This broad view of the design system usually entails a less detailed level of representation. Indeed, it is pointless to model extremely detailed effects, such as turning movements at intersections or flows on minor roads, because they are not significant for the evaluation of the project under study.
Feasibility Studies
Feasibility studies are assessments of the technical possibility, economic worth, pri- ority level, and execution mode of individual transportation projects. Project defin- ition is generally derived from a higher-level reference scheme, such as a strategic plan, that identifies new connections needed in the transportation network.
Technical and economic feasibility studies of transportation projects usually re- quire the formulation of project alternatives in terms of their performance and functional characteristics (such as layout, connections, capacity, service perfor- mance, type and characteristics of vehicles and technologies, and prices). Alter- native projects, including the do-nothing or reference solution, are then evaluated from the functional, economic, and financial points of view, in the context of differ- ent transportation and activity system scenarios. The analysis time horizon in this case is usually long-term and the geographic scale varies from urban to regional or national according to the kind of project to be assessed. The definition and func- tional characteristics of the larger system can be analyzed and modeled at levels of
26 1 Modeling Transportation Systems: Preliminary Concepts and Application Areas detail that vary according to the intensity of the interactions with the project being studied. For example, a denser zoning system can be adopted around the proposed alignment of a new railway. Whatever the case, the system must be modeled con- sidering the travel demand for and supply of all transportation modes.
There are many examples of feasibility studies. Some studies are aimed at as- sessing the financial worth of private capital investments in facilities and/or trans- portation services (project financing). In this case, forecasts of travel demand, user flows, and revenues are of special interest, as are the external conditions under which expected demand and financial returns can be obtained.
Tactical Planning
Short- or medium-term tactical planning involves decisions about projects requir- ing limited resources, usually assuming minor or no changes in existing facilities.
Urban traffic plans or public transportation plans are examples of tactical plans un- dertaken by public agencies. The design of scheduling or pricing policies for air or rail services are examples of tactical plans carried out from the operators’ point of view.
Of primary interest in this context are evaluations of the technical and functional impacts of the project, as well as analysis of its financial performance in terms of operating costs and traffic revenues. These analyses might be accompanied by an economic appraisal, although this is often simplified. For these applications, the socioeconomic scenario is usually taken as given. In practice, it is also assumed that the level and spatial distribution of travel demand are unaffected by the projects, whereas variations in modal split and flows on the project networks are explicitly modeled. In some cases, a single transportation mode is examined in the context of the overall system; the effects of intermodal competition are then considered only through the level of demand of the mode considered (elasticity analysis), without explicit representation of the network and service characteristics of the competing modes.
Operations Management Programs
Short-term operations management programs generally focus on particular aspects of the operations of individual transportation modes, optimizing the use of available resources usually from a company or agency point of view. The design of traffic signal control plans, preparation of transit timetables, and organization of factors necessary for producing transportation services (e.g., assignment of vehicles to lines and travel staff to work shifts) are examples of operations management programs.
In this case, the study is usually limited to a single mode and assumes that the modal demand is fixed. For example, only the road subsystem (network and de- mand) is considered in designing a traffic-signal control scheme. If necessary, net- work and assignment models described in later chapters can be integrated with de- tailed microsimulation models. Furthermore, the design phase can be carried out with the support of supply design models similar to those described in Chap. 9.