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The Traffic Assignment Problem

Models & Methods

Dover Publications, Inc.

Urban traffic planning

1.1 Introduction

A significant amount of the activity in an urban area concerns the movement of people and goods between different locations in the transportation infrastructure, and a smooth and efficient transportation system is essential for the economic health and the quality of life within the urban region. When analyzing the present infrastructure for future investments and operating policies, a careful study of the transportation system is therefore among the most important components of the planning process.

The decades following World War II have seen an enormous increase in the demand for transportation. A vast majority of this increase is accounted for by the development of personal transport, which has its roots in the urbanization and the rising standards of living. The increase of mobility has, however, also brought many serious problems into urban regions, such as pollution, increased accident rates, unwanted social effects on urban life due to highway expansion, and an inefficient use of the transportation system because of high congestion.

In transportation planning studies alterations of the existing transportation systems are evaluated with the objective of alleviating the above mentioned problems (among others), while also utilizing the full range of transport modes available.

Urban transportation planning has been an evolutionary process. Its beginnings may be traced to the home-interview studies conducted in more than 100 cities in the United States during the decade following the end of World War II. The concept of small sample interviews was then combined with cordon line surveys in order to derive patterns of urban travel. Future traffic usage of urban highway projects was predicted by manually assigning selected origin-destination (O-D) movements to the routes being planned. In the early 1950s there were studies investigating land use and traffic relationships because better estimating methods were needed in order to forecast the travel in the design year. Methods of forecasting future population and its distribution, trip generation analysis relating travel to underlying household characteristics (car ownerships, etc.), and planning for networks instead of single routes were introduced at this time. Improved procedures were facilitated by the growing use of punch card data processing systems and later by the increasing capabilities of electronic computers. The latter permitted greater sophistication in transportation planning because they permitted the examination of more alternatives. The "modelling" of future land-use plans and future highway and transit systems was combined with more elegant methods of evaluation. Criteria for determining if plans met community objectives (a concept itself not generally introduced until the mid-1960s) could be increasingly quantified.

The first transportation studies made concerned only highway traffic, and saw the problem as being that of providing enough capacity for the estimated future demand for personal transport. Since the 1950s, however, it has been realized that transportation is not an isolated activity; indeed, the demand for travel facilities is a function of human land use activity and, conversely, the provision of transport facilities stimulates land use activity. This development can also be seen in the Federal-Aid Highway Act of 1962, which states that federally assisted highway projects must be "... based on a continuing comprehensive transportation planning process carried on cooperatively by states and local communities ...". As a result of these findings, recent transportation studies form integrated parts of the overall planning process, and the so called 3C philosophy of continuing, comprehensive, and cooperative urban transportation planning characterizes the current status of the process. Transport planners focus more on improving public transport, as an alternative to the auto mode, in order to reduce highway congestion.

The transportation system is very complex, and its performance depends on decisions made on many levels of society (the goals and purposes of which may be in conflict with each other). The process of evaluating, designing and managing such a system can therefore not be carried out without the aid of properly formulated models.

Depending on the purpose of the transportation study, models may concern different components of the transportation system (land use patterns, control policies, trip generation and distribution, etc.), different levels of aggregation of the physical reality (macroscopic or microscopic models), different planning horizons (from the use in realtime traffic management systems up to 20 year forecasts), and be based on different modelling principles (statistical models, optimization models, simulation models).

As the understanding of the transportation system has grown, together with the increase in availability of computational tools for its analysis, the planning problem has become more complex. The costs have also increased, due partly to the increase in costs for the inventory stage, and also because several more alternatives are tested. However, viewing these costs against the scale of the plans they produce, the planning costs are less than one percent of the total ([37]).

1.2 The transportation planning process

The basis of the modelling of transportation problems is a set of assumptions, the most important ones being that travel patterns are tangible, stable, and predictable, and that the demand for transportation is directly related to the distribution and intensity of land uses, which are capable of being accurately determined for some future date ([130]).

Domencich and McFadden [264, Chap. 1] provide one list of criteria which a demand-based transportation planning model should meet in order to be a practical tool for policy analysis: it should be sensitive to transportation policy, so that the effects of policy alternatives can be forecast; it should be causal, establishing the behavioural link between the attributes of the transportation system and the decisions of the individual. This leads to the investigation of behavioural models of individual travel demand. Further, it should be flexible, allowing application to a wide variety of planning problems without major data collection and calibration costs; it should be transferable from one urban setting to another, allowing reuse without expensive reestimation in each new setting; finally, it should be efficient, in terms of providing maximum forecasting accuracy per monetary unit spent on data collection.

The traditional approach to transportation planning is to identify a number of simple submodels of the whole system, which are then analyzed separately, and most often in sequence. This transportation planning process can be divided into the following steps:

Step 1 (Organization and goal definition) The first stage of the process includes obtaining agreement on the funding, participation, and organizational form, setting up the committee structure, and arranging for staffing the study. Statements of goals and objectives of the study are also made.

Step 2(Base year inventory) At this stage the data that may be relevant to the analysis of the transportation system is collected. It includes an inventory of existing transportation facilities and their characteristics, existing travel patterns determined through origin-destination surveys and traffic measurements, and planning factors, such as land use, income distribution, neighbourhood structure, and types of employment. It also includes the collection of historical data for trend analyses, such as population growth and car ownership.

Step 3(Model analysis) The purpose of this phase is to establish relations among various quantities measured in Step 2, and to calibrate these relations for the base year. The relations are usually determined through the use of the following mathematical models, which are considered in sequence, and where the output from one model is input to the next.

(a) (Trip generation) This model is used to determine the number of trips originating and terminating in different zones of the study area. These numbers, which are sometimes called production and attraction numbers, are usually defined as functions of socio-economic, locational and land use characteristics of the zone in question, and are divided into different categories of purpose, such as work and recreational trips.

(b) (Trip distribution) At this step, formulas are derived to describe the allocation of trips from a point of origin to the destination zones. These formulas are typically defined as functions of the production and attraction numbers of the different zones, produced in step (a), and of the travel costs between them. In some models, traffic counts are used when determining the trip matrix.

(c)(Modal split) This model determines the portion of the total number of trips made between an origin and destination using different transport modes, the two most commonly considered being cars and public transit. The portions of trips in an origin-destination relation is normally derived from relative travel times and costs between modes, and also, in some cases, from the socio-economic and land use characteristics of the origin and destination, respectively.

(d) (Traffic assignment) In this model, the origin-destination trips are allocated to routes in the transportation network, in order to estimate the traffic volumes and travel times on the roads as functions of the network characteristics. The underlying behavioural principle in the choice of route is normally that travellers try to minimize their own travel costs.

Step 4(Travel forecast) Based on the data collected in Step 1 and trend analyses, future land use, population distribution, etc., are predicted for a design year. The models developed and calibrated in Step 3 are then used to estimate the generation and distribution of trips on the future transportation network.

Step 5(Network evaluation) If alternative future transportation networks and facilities are proposed, in this step costs and benefits are compared between their predicted flow patterns, in order to provide a basis for an economic evaluation of the proposed new facilities.

In order to achieve a consistent output the steps of the planning process must be repeated. Indeed, the travel costs of the future transportation network given by Step 4 influence the trip distribution, and even the projected land use and trip generation! This inconsistency problem can (at least partially) be alleviated by considering parts of the process simultaneously. Recent research efforts are being made in this direction.

In the sequel, we shall study the different parts of the transportation planning process in more detail, and outline the most common methods employed for their solution. We will here concentrate on the models and methods developed within transportation planning studies, and describe those developed through academic research in subsequent parts of the book.

1.3 Organization and goal definition

It is important for the result of the transportation study to establish goals and objectives early in the process, since these will guide the evaluations towards conformity with the desire of the community ([893, 130]). Traditionally, as already mentioned, the main objective of the transportation study has been to evaluate alternative highway constructions for increased personal transport capacity ([862]). Other goals considered have also mainly been orientated toward traffic functional aspects, such as an increased safety, a saving of travel time, a reduction of operating costs, and an increase in efficiency and mobility. It is only during the last 25 years that environmental aspects and the transit alternative have been considered essential elements of the transportation study. See [984, 37] for a more detailed description of the goal setting.

The topology of the study area, the population distribution and many other socioeconomic factors vary from study to study. The form of the study may therefore differ significantly among different countries and regions.

Studies may be of long-range type, in which case the most important questions to be answered deals with the density and configuration of the future transportation system. Short-term plans may include immediate-action programs for arterial improvements. The scale of the study may also differ; some plans include proposals for new facilities, such as parking, terminals, and transit lines, while others may describe highway locations with ramp connections pinpointed, or only deal with single corridors.

The personnel organization of the study can also have several different forms. The Transportation and Traffic Engineering Handbook [37, pp. 517-518] lists the following alternatives: A centralized state staff may be an existing agency or a new department incorporating the necessary multidisciplinary talents. The Chicago Area Transportation Study [170], established as an ad hoc joint effort and responsive to a multiagency board, illustrates the use of a semi-independent organization. A council of governments is a study organization which may be created under a council made up of elected representatives of communities within the region. Established planning bodies for metropolitan regions are sometimes the organizations housing the transportation planning staff. In a contract study organization consultants under the supervision and monitoring of either a state representative or local study director perform all or some of the stages in the planning process. The procedure has been used extensively in the U.S. ([882]).

Regardless of the organization structure, an additional organization must be appointed to ensure that the activity of the planning staff agrees with the goals and objectives set up ([489]). This organization could comprise of the following committees ([37, pp. 518-520]):

The policy committee includes representatives of agencies participating financially in the study, as well as officials and executives of local and regional planning organizations. The function of this committee is to provide budget control, establish regulations for study personnel, supervise technical matters, establish objectives, assist in the plan development, and recommend a final plan ([692]). The technical committee includes technical personnel from agencies represented on the policy committee, and sometimes also from other local agencies. The function of this committee is to review and evaluate study methods, assist in developing alternative plans, perform technical evaluations, coordinate technical service contributions of participating agencies, and enlist the interest of local agencies in the planning process. The composition and function of the citizens advisory committee vary with the size of the study area, and the interest in the study objectives of the communications media. The committee provides the policy committee with information on public thinking, and can thus assist in the definition of planning goals and objectives, improve public understanding of the planning process, and build support for plan implementation.

1.4 Base year inventory

The inventory stage can be divided into four categories:

(1)(Transportation facilities) Here, the study area is defined, and divided into sectors, districts and zones. The physical network is represented by a graph, with streets and road sections represented by links (or arcs), and intersections and trip origins and destinations by nodes. The boundary of the study area, referred to as the external cordon, is chosen to approximate the commuter-shed associated with the urban centre. The zones represent aggregates of trips and socio-economic conditions; the choice of zones is very important, since the number of zones determines the complexity of the study, and the wrong choice of zone size and distribution would obscure a lot of the information in the data collected ([130, 264, 805]). The number of zones ranges typically from 10 to 1000, and their sizes from a few blocks to several square kilometers. Whenever zones are small, their locations may be defined by single points in the network description, the so called zone centroid nodes.

Next, the characteristics of the existing transportation network are colleted; data includes measurements of traffic flows, speeds, travel times (or delays), link lengths, capacities, and the quality of transit service. There are many techniques for measuring these performance characteristics; some of the data required is recorded automatically by many traffic control systems, other information can be obtained from census data ([130]).

(2)(Travel patterns) Data relating to the present-day movement between zones is collected at this stage. Traffic pattern data is required for all combinations of external and internal movements. The data may be divided into trip mode and purpose. The goal of this data collecting is to estimate the number of trips made between zones within the study area, and the number of trips passing through, into or out of the area.

Movements through the area and external-internal movements are surveyed at the external cordon, and possibly at an internal cordon or screen line; this is done by manual or automatic counts. Internal-external movements are surveyed in the home-interview study and at the external cordon, while internal movements are surveyed by home-interview studies and, sometimes in addition to check, by an internal cordon or screen line survey.

The size of the sample to be interviewed depends on the total population of the area, the degree of accuracy required, and sometimes on the density of the population. The recommended sample sizes for home interviews are between 4 and 25 percent of the total population ([130]). For roadside interviews, the sample can be based on time or volume clustering, or could vary among classes of vehicles ([52, Chap. 4]).

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Excerpted from The Traffic Assignment Problem by Michael Patriksson. Copyright © 2015 Michael Patriksson. Excerpted by permission of Dover Publications, Inc..
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