Field Test of a Method for Finding Consistent Route Flows and Multiple-Class Link Flows in Road Traffic Assignments
Road traffic assignment, e.g. forecasting of route and link is one of the most intensively applied tools used by transportation planning practitioners. Transportation planning practitioners routinely perform select link analyses with their travel forecasting software systems in an effort to identify the beneficiaries of proposed network improvements. Despite 50 years of development and deployment, first by public agencies and academic scholars, and later by commercial software developers, current travel forecasting software systems do not address several specific needs faced by practitioners: 1. consistent forecasts of multiple-class link flows within the solution for a single network, or across solutions for two or more alternative networks;2. consistent forecasts of route flows by OD pair within a single network or across several networks;3. consistent forecasts of route flows for use in traffic micro-simulation models. Traffic Assignment by Paired Alternative Segments, or TAPAS, a new algorithm stemming from the research of Bar-Gera (2010), offers a practical opportunity to address these problems. Three general types of improvements are expected from use of consistent route and multiple-class link flows. 1. Improved understanding of the origin-destination movements contained in selected link flows allows transportation planners to estimate who benefits from link improvements so that transportation improvement programs can be better evaluated with respect to social equity and environmental justice criteria. 2. Consistent representation of the types of vehicles on links is important for predicting emissions, where multiple trip matrices correspond to various categories of vehicles with different emission characteristics. Similarly, assigning truck trip matrices by type separately from private autos will yield consistent truck flows on links, which would not only improve goods movement analyses but allow better measurement of the consequences of heavy truck flows on links, such as their impacts on noise and safety. 3. Road pricing schemes are being considered by many MPOs and state DOTs. An important component in their evaluation is to determine who will pay the tolls, a task requiring consistent multiple-class link flows. This project centered around two activities: 1. Deployment of the TAPAS algorithm: Six practitioners used TAPAS, together with their practitioner software systems, to perform select link and related analyses. The case studies, collaborators, number of user classes, software and method used are shown in Table 1.1. Table 1.1 Summary of the case studies
Case Study
Collaborator
Classes
Software
Method
MAG
V. Livshits
1
TransCAD
Link-based
PAG
A. Sun
1
TransCAD
Link-based
DVRPC
C. Puchalsky
1
VISUM
Route-based
PSRC-PTV
C. Joshi
2
VISUM
Route-based
PSRC-RST
R. Tung
2
EMME
Link-based
ODOT
R. Anderson
2
CUBE
Link-based
2. Comparison of results among alternative software systems: To develop needed tools and to prepare for the efforts of practitioners, the project team undertook its own case study with each of the practitioner software systems using historical data files from the Chicago Area Transportation Study and OD matrices devised previously in their research. Three main types of comparisons were conducted: 1. select link analysis, which is widely used in practice;2. direct evaluation of proportionality by analysis of equal-cost pairs of alternative segments; 3. class-specific link and segment flows. Main findings regarding commercial link-based tools: While slow convergence is costly, the findings available thus far suggest that link-based tools do provide approximately proportional solutions in terms of route flows and multiple-class link flows. This capability was not known at the outset of this project. Main findings regarding commercial quick-precision tools: While recognizing the advantage of quick-precision tools, caution is advised regarding analyses related to route flows and multiple-class link flows based on solutions produced by these methods. Main findings regarding TAPAS: TAPAS is a quick-precision assignment tool providing route flows and two-class link flows that satisfy the assumption of proportionality.