Travel Demand Forecasting
Manual 1
Traffic Assignment Procedures
Ohio Department of Transportation
Division of Planning
Office of Technical Services
by
Gregory Giaimo, P.E.
August 2001
Table of Contents
Prologue1
Chapter I. Introduction2
Purpose2
Input2
Overview of Process3
Assignment Types3
History4
Chapter II. Transportation Network Considerations7
Traffic Analysis Zones7
Zone-Network Relationship7
Transportation Network Level of Detail8
Centroid Coding9
Impedance Coding9
Travel Time/Speed9
Distance10
Turn Penalties/Prohibitors11
Capacities & Related Link Data11
Counts12
Other Network Information12
External Stations13
Chapter III. Assignment Techniques14
Hourly vs. 24 Hour Assignments14
All or Nothing Assignment15
The BPR Equation19
Iterative Capacity Restrained Assignment22
Equilibrium Assignment23
Feedback Loops25
Count Restraintand Count Based Trip Table Estimation25
Chapter IV. Assignment Limitations, Checks & Refinements26
Assignment Limitations26
Assignment Checks/Validation27
Plot Check28
Percent Root Mean Square Error Check28
Vehicle Miles Traveled Check31
Screen Line Check32
Select Link Check33
Assignment Refinements33
Chapter V. Tranplan Modeling36
Network Building36
All or Nothing Assignment37
Iterative Capacity Restraint Assignment38
Equilibrium Assignment39
Assignment Checking41
Appendix A. Guidelines Summary43
Appendix B. Network Format65
Appendix C. Speed Tables68
Appendix D. BPR Parameters70
Bibliography72
List of Figures
Figure 1. Path Building Example16
Figure 2. Vine Paths vs. Tree Paths17
Figure 3. Original BPR Curve19
Figure 4. BPR Exponent Variation21
Figure 5. New BPR Curves by Link Group21
Figure 6. Expected Coefficient of Variation in Daily Count Volume27
Figure 7. Allowable Percent Root Mean Square Error30
Figure 8. Maximum Desirable Deviation in Screenline Volume32
1
Prologue
This document is one in a series developed by the ODOT Modeling & Forecasting Section relating to travel demand forecasting procedures. The current version represents the first update of the original version written in 1996. While the first Traffic Assignment Procedures manual merely attempted to document past practice, the current manual expands upon past practice to add new techniques that are recommended (37) for the new generation of models to be developed based on the 2000 Census and Household Travel Surveys. In addition to this document, another on Highway Network Coding Procedures (1) has been developed and will soon be updated to be consistent with the content of the present manual.. Future manuals on Transit Network Coding Procedures and Trip Table Synthesis (Demand Modeling) Procedures may also be released at some time. Taken together these documents represent a complete set of guidelines for using four step travel demand forecasting models in Ohio. Other procedures dealing with pre and post processing to the four step process have been deliberately omitted from this series and are dealt with individually as necessary. The key post processing element being the computation of congestion and emissions measure using ODOT's CMAQT program (2) while pre-processing typically involves running ODOT's capacity (18), speed (3) and turn penalty calculators (24).
Throughout this document, Ohio procedural guidelines are underlined to make them easily distinguished from the accompanying descriptive information. These guidelines are summarized in Appendix A.
I. Introduction
Traffic assignment is defined as the process of allocating a set of trip interchanges to a specific transportation system based on specific criteria as to choice of route. The choice of route criteria is that the travel impedance through the transportation system be minimized for a given origin-destination pair. The output of traffic assignment is "an estimate of user volumes on each segment of a transportation network as well as the turning movements at each intersection of the transportation network".(4)
Purpose
The purposes of performing a traffic assignment have been variously given in a number of sources (5), (6) as:
1.To determine deficiencies in the existing system
2.To assist in the development of the future transportation system
3.To develop construction priorities
4.To provide highway designers with design-hour traffic volumes
In addition to these, many other uses of traffic assignments have developed such as air quality analysis, congestion management analysis and major investment studies. The number of uses for traffic assignments are many and can be summed up simply as: traffic assignments are used whenever a prediction of transportation segment volumes (highway vehicle volumes or transit ridership) are needed.
Input
There are two basic inputs to traffic assignment: a matrix of trip interchanges and a computerized description of the transportation system.
Creation of the trip interchange matrix or trip table is a function of travel demand modeling and will not be covered here. The trip table is a matrix of volumes of vehicles or persons that desire to move from one area to another. Each row of the matrix represents those trips wishing to go from one area to all other areas while each column represents those trips wishing to come to one area from all others. The matrix as a whole allows for the interaction of trips between all areas.
These areas are known as traffic analysis zones. Traffic analysis zones are merely geographic areas used to aggregate travel behavior into manageable units. Travel actually occurs between discrete traffic generators such as homes and businesses. To model the interactions between all homes and businesses in an area would be very difficult, so travel desires are aggregated into traffic analysis zones. The use of traffic analysis zones to aggregate travel behavior is a fundamental concept of travel demand forecasting as it is currently practiced. The delineation of these zones will be discussed in more detail later as well as in the Trip Table Synthesis Procedures Document.
A trip table can be obtained in one of two ways, by actual measurement via an origin-destination survey or through synthesis with travel demand models (trip generation, trip distribution and mode split models). These synthesis models are estimated/calibrated using an origin-destination survey. In practice, it is impossible to obtain a statistically significant trip table from an origin-destination survey for an urban area with a reasonably detailed transportation network. Therefore, models are the only viable way to obtain this trip table. Regardless how it is obtained, any trip table can be assigned to a transportation network. Which trip tables are used when and why will be discussed later in this document.
The second input to traffic assignment is the computerized transportation network. There are two types of transportation network used in Ohio, the highway network and the transit network. Development of the highway network is discussed in Highway Network Coding Procedures while development of the transit network is discussed in (27) and (28). A transportation network is represented by a series of links and nodes. A link represents a segment of street and contains data on the characteristics of that street, a node represents an intersection or other point where conditions on the street change. Additionally, transit networks also contain lines which are lists of nodes through which a given transit route passes. A transit line is a path in its own right, which all transit minimum time paths are restricted to follow. A given transit passenger may transfer between transit lines to reach his destination, but he must always follow the prescribed lines. Special nodes called centroid nodes or just centroids, are coded into a highway network for each traffic analysis zones. These centroids are connected to the highway network through special links called centroid connectors. A centroid connector is a fictitious street segment which may represent a number of local streets and access points which are not otherwise part of the transportation network. Centroids are used to load trips from the trip table onto the transportation network and are thus analogous to sources and sinks in say a pipe flow network.
The most basic information about a transportation network that is needed for traffic assignment purposes is the connectivity between links and a measure of impedance on that link. Connectivity is indicated on a link by listing the nodes that it connects. All links that meet at a given node are said to be connected. A link is, in fact, identified by this node to node listing. Thus a link that connects node 52 to node 121 is referred to as link 52-121. Travel impedance is typically indicated by the time to traverse the link but can also include the distance traveled and the monetary cost. Travel time includes all delays due to intersections and other conditions on the link unless intersection delay is being explicitly modeled at the intersection nodes. The travel time can be indicated in one of two ways, either by indicating it directly or by giving the average speed that can be maintained across the link as well as the distance of the link. Impedance coding and other network parameters will be discussed later.
Overview of Process
Traffic assignment is basically a two step procedure. First minimum impedance paths are determined between all traffic analysis zones using the network impedance discussed above. Paths are not built one at a time for each zonal interchange but instead are built for each origin zone in such a way that all overlapping portions of the zone to zone paths from that origin only need be built once. For example, if a given centroid had 4 centroid connectors and the network as a whole had 500 zones, at most only 4 calculations would be required in finding the first link of the path instead of 500 if each path were calculated individually.
The second step is to load the trip table to the network by utilizing the minimum impedance paths. Each trip interchange in the trip table is individually loaded to the path between the two zones it represents. The volume of the trip interchange is accumulated on each link that belongs to that path. Each zonal interchange whose path crosses a given link adds its volume to the link in turn until a final traffic assignment volume is accumulated after the entire trip table has been loaded. Trip loading occurs backwards from the destination zone to the origin zone. This is because the path set from the origin (often called a tree or vine) emanates out from the origin and goes to each other node in the network exactly once. Loading backwards ensures that you eventually reach the origin node. An analogy can be drawn from a watershed where the origin is the mouth of the primary river and the destinations are at the headwaters of the various tributaries. Going downstream from the destinations will always eventually get you to the origin while going upstream from the origin might get you to any of the destinations. Paths are built upstream to ensure all destinations are reached and loaded downstream to ensure the origin is reached.
Assignment Types
In Ohio there are four basic types of traffic assignment used. These are:
1.Base year calibration/validation assignment
2.Existing year assignment (when different from base year)
3.Future year no-build assignment
4.Future year build assignment
All other assignment types can be classified as one of these four. The base year validation assignment is intended to be made every 10 years to correspond to the availability of census data. Unlike other assignments this assignment involves collection and coding of ground counts into the network to check the accuracy of the modeling process. Much additional work goes into the creation of trip tables for the base year validation. The model checks discussed in Chapter IV. of this document are then used to validate and adjust the network model until it adequately represents ground counts. One very important point that should be made is that the final base year validation modeling chain must be reproduced exactly for forecast year assignments. This condition prohibits the use of count restraining or count based trip table estimation and requires the use of capacity restraining in the assignment process. Count and capacity restraint will be discussed more later in this document.
Because the base year assignment is only made once every 10 years, it is sometimes necessary to have an existing year assignment in the intermediate years. This assignment can be used to check the forecast assumptions and to serve as the basis of further analysis.
The future year no-build and build assignments utilize forecasted trip tables to evaluate the system effects of transportation improvements. The build and no-build scenarios are each assigned a forecasted trip table and the differences in various measures of congestion are analyzed. For Long Range Planning purposes, the no-build network is the existing + committed network (E+C). This network represents currently existing conditions plus those projects already committed for building where in the past committed variously meant projects which are programmed, projects that have gone into the environmental process or projects appearing in the TIP. ODOT recommends the E+C networks be equated to the TIP network. The TIP network which is currently used primarily for air quality analysis contains all projects in the TIP plus significant local projects not in the TIP. Equating these networks provides a measure of consistence in the process and results in less networks to be maintained. The build scenario involves the E+C plus all projects that will appear in the long range plan. Several possible alternative LRP's should be analyzed and the best selected for implementation. This of course assumes that the alternatives are all feasible given fiscal constraint, the political climate and other factors.
Build/no-build analysis is also used for various other purposes including congestion management analysis and project/corridor specific planning studies. For analysis, the year is determined and networks representing proposed conditions that year (both with and without the subject projects) are coded. A trip table for that year is created using the trip table synthesis procedures and assigned. Plots of the assigned volumes and volume to capacity ratio are then analyzed to determine project benefits which can then be used in benefit cost analysis. Other analyses use only build scenarios such as air quality budget analysis and design year traffic analysis.
History
The beginnings of traffic assignment can be traced back to the introduction of origin-destination surveys. These surveys produce trip tables which show the trips people make independent of route choice. To properly analyze the effects of this trip table a method was needed to determine which routes the trips in the trip table would use to move from origin to destination. Early methods used manual techniques to determine the routes people would use between each origin and destination, computers were only used to accumulate trips from the trip table on the manually determined routes. Other work was in the area of diversion analysis in which various methods were proposed for determining how many trips would divert from current routes to a proposed route. These methods utilized diversion curves which expressed the number of trips diverting between routes as a function of travel time ratio. These methods were only conducted at the corridor level and often gave unpredictable results.
The most critical aspect of a traffic assignment program is that of route selection or path building. Attempts to develop a computerized program for system wide traffic assignment had been blocked for some time by a need to find an efficient route selection algorithm. According to USDOT's Traffic Assignment Manual (4), "The breakthrough in the network path determination came from work undertaken to solve the route selection problem of the telephone systems for direct dialing of long distance telephone calls...Two papers published in 1957 provided the impetus for computerized route selection process for traffic assignment: The Shortest Path Through A Maze, by E.F. Moore(7); The Shortest Route Problem, by G. Dantzig.(8)" Transportation agencies were quick to exploit these methods and by 1960 work by Chicago, Washington D.C. Detroit, California and Minnesota had resulted in the first Bureau of Public Roads Traffic Assignment Battery.
One early development came when researchers realized that there is a relationship between volume and travel time (known as a speed (time) - volume curve, the most commonly used is know as the BPR curve) as shown in the Highway Capacity Manual (9-11). In an attempt to reflect this relationship in assignments and to produce a "multi-path" diversion type effect between zones capacity restraint algorithms were incorporated, first by the staff of the Chicago area study. Capacity restraint is an iterative process whereby an assignment is made and the link impedances are adjusted based on a speed-volume curve. The assignment is then redone and impedance readjusted. The final capacity restrained assignment is not used alone but rather weighted with previous iterations to produce the final weighted capacity restrained assignment. The Traffic Assignment Manual has an excellent discussion of Capacity Restraint which states:
"There is a relationship between speed and volume on all types of facilities, both for interrupted and uninterrupted flow. On facilities such as freeways, there is a constant decrease in speed with increase in traffic volume up to a point of critical density. Beyond this point, however, both volume and speed decrease with an increase in density. The situation is similar at interrupted flow type facilities. Here, however, speed is influenced by external influences such as signal progression, speed limits, and the conditions on adjacent sections...The traffic assignment process assigns trips in accordance with impedances coded on each network segment. These impedances are usually travel time or some derivative of time. The assignment process results in the traffic load on each network segment. Since there is a very direct relationship between travel time (or speed) on a section and the volume on the section, a process is necessary to allow consideration of this relationship. This process is referred to as capacity restraint. Specifically, the capacity restraint process attempts to bring the assigned volume, the capacity of a facility, and the related speed into the proper balance. There are several problems in the application of speed-flow relations in the assignment process. Most important is the assignment of trips for some extended period of time, such as a day...Most critical flow problems actually occur over shorter time spans. Secondly, the assignment process may load a facility far in excess of capacity based upon some originally coded speed. Observed conditions are limited to some maximum capacity. Because of this, capacity restraint functions are theoretical extensions beyond some critical capacity point."