Section 1.0 INTRODUCTION. Section 2.0 MODEL ARCHITECTURE RECOMMENDATIONS, PHASE I, TIER I - PASSENGER CAR AND TRUCK..

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1 TABLE OF CONTENTS Section 1.0 INTRODUCTION Section 2.0 MODEL ARCHITECTURE RECOMMENDATIONS, PHASE I, TIER I - PASSENGER CAR AND TRUCK.. Section 2.1 General Software and Forecast Year Recommendations. Section Software Platform... Section Software Interface... Section Base and Forecast Years Section 2.2 General Model Outline. Section 2.3 Model Structure Section Highway Network Section Zone System Section External Networks and Stations. Section Socioeconomic Data Section Trip Generation... Section Trip Distribution. Section Mode Split Section Traffic Assignment.. Section Truck Model Section External-External and External-Internal Trips.. Section Calibration and Validation... Section Post Processing.. Section Graphical User Interface... Section Model Documentation... Section Typical Policy-Based Sensitivity Analysis. Section 3.0 MODEL ARCHITECTURE RECOMMENDATIONS, PHASE II, TIER II RAIL FREIGHT AND COMMODITY FREIGHT FLOW... Section 3.1 Network Development.. Section 3.2 Commodity Forecast Section 3.3 Commodity Movements... Section 3.4 Freight Mode Split Models.. Section 3.5 Commodity Density and Load Weights.. Section 3.6 Traffic Assignment... SUMMARY

2 LIST OF TABLES Table 1: Master Network Structure.... Table 2: Sample Capacity Lookup Table for Statewide Traffic Model..... Table 3A: Home-Based Work Production Rates (Urban)... Table 3B: Home-Based Work Production Rates (Rural)... Table 4: Attraction Rates... Table 5: Typical Auto Occupancy Rates... Table 6: Typical External-Internal Production Approach.... Table 7: Traffic Count by Volume Class with % Root Mean Square LIST OF FIGURES Figure 1: Phase I and Phase II Components..... Figure 2: Model Architecture Process Flow.. Figure 3: Typical TAZ Structure Geographic Units for Statewide Travel Models... Figure 4: Trip Length Frequency Distribution for Medium and Heavy Trucks.. Figure 5: Example of a National Zone System to Serve a Statewide Model..... Figure 6: Percent Deviation for Total Vehicle Calibration Links Figure 7: Draft Screenlines for Iowa Statewide Model.. Figure 8: Draft Set of Representative Cutlines in Iowa Figure 9: GUI - Scenario Manager Figure 10: GUI Link Speed Inputs.... Figure 11: GUI Capacity Inputs.... Figure 12: Assignment Parameters APPENDIX A: Existing Data Sources and Evolving Ideas for Data Resources

3 1.0 INTRODUCTION During the Phase I Needs Assessment task of the Iowa DOT Statewide Model Study, the Wilbur Smith team, in cooperation with Iowa DOT Systems Planning staff, conducted a series of informational meetings, stakeholder workshops, and executive project steering committee (EPSC) meetings to determine the highest potential uses for the proposed Iowa Statewide Model. Data sources were also reviewed and identified. Based on the Final Needs Assessment Report, which is Tier I of the current effort, the Iowa Statewide Model will be used to: A. Evaluate the Impact of Network Changes 1. Run Test Scenario Analyses 2. Evaluate Capacity Increases a. Additional Lanes b. Roadway Improvements c. New Roads B. Support Interchange Justification Reports (non-mpo areas) C. Evaluate River Crossings and Bridges (emphasis on state lines) D. Monitor Major County Roads (track traffic changes over time) E. Estimate Future Loadings (trucks) for Pavement Design F. Conduct Major Corridor Analyses (multi-county) G. Provide Estimates for Performance Measures. Based on the Statewide Travel Model capabilities that have been identified by the Iowa DOT Systems Planning staff, the EPSC, and the stakeholders, and with the knowledge that financial resources are constrained, the WSA Team recommends that both a passenger car model and a truck model be developed in the first phase of model development, Phase II, Tier I (See the Final Needs Assessment Report) and Figure 1. These first two model components will provide a solid, four-step statewide model for Iowa that is practical to use and consistent in producing reliable forecasts, and will meet the initial uses noted above. As financial resources become available, it is recommended that Phase II, Tier II be developed, to include the development of a Statewide Rail Freight model and a Commodity/Freight Flow model. Where possible, the Phase II, Tier I effort will preserve the envelope, that is, design the initial model components so that enhancement or extension can be readily introduced without a complete retrofit. As an example, truck traffic can be forecast without a commodity freight flow sub-model. The truck model should be designed so that the commodity flow model can be integrated into the initial model when Phase II, Tier II is begun. Careful attention should be given to insuring that the network and the socio-economic data when initially designed will support both a truck model as well as a commodity- based model. 3

4 Figure 1: Phase I and Phase II Components 4

5 The Needs Assessment Report recommended that the Iowa Statewide Model be able to perform the following analyses to meet the initial uses noted above, as part of the Phase II model development: Tier I 1. Traffic Forecasting (Automobile and Truck) 2. Statewide and Regional Corridor Analysis 3. Policy Evaluations (Finance, Funding, and Project Prioritization) 4. Rural Freeway Interchange Evaluations 5. MPO External and Through Trip Analysis 6. Major Stateline Crossing Analysis (Bridge and Highways) 7. Special Generators 8. Safety Analysis (investigate the use of PLANSAF) Tier II 9. Statewide Rail Freight Analysis 10. Commodity/Freight Flow Analysis (non-modal) 11. Commodity Policy Evaluations 12. Statewide/Regional Rail Freight Corridor Evaluations 13. Major Rail Terminal Evaluations 14. Commodity Information Management System Using the identified model purposes, the data availability, and minimizing the need for conducting household and traffic surveys, the following general software, model architecture and forecast year are recommended. 2.0 MODEL ARCHITECTURE RECOMMENDATIONS, PHASE I, TIER I - PASSENGER CAR AND TRUCK 2.1 General Software and Forecast Year Recommendations Software Platform The software platform for the Iowa Statewide Model will be based in TransCAD. The latest TransCAD version will be used when Phase II is undertaken. Either Version 4.8 (the most currently available from Caliper Corporation), or 5.0, will be used. Within each release, Caliper Corporation also uses a sub-release identification scheme, known as a Build. Since both the release and the build version contribute to the stability of the GISDK macro language within TransCAD, it is recommended that both of these items be established at the start of the study. The consultant team will obtain a copy of Version 5.0 and test it in early All things being equal, the latest version of TransCAD is to be used. Since the Iowa DOT Office of Systems Planning currently possess multiple TransCAD licenses, this group will have their version of TransCAD updated as part of their maintenance agreement. An additional copy of TransCAD will be purchased specifically for the statewide model (the estimated cost is $5,000 with a yearly maintenance fee of $1,000 after the first year). It is recommended that a dedicated statewide model laptop computer be set up at the Iowa DOT s Office of Systems 5

6 Planning. As part of the contract costs, the consulting team will purchase a laptop computer for the project. The purchase of the computer will be coordinated with the Iowa DOT Systems Planning and Information Technology staff to ensure compatibility with the Departments computer network and operating system Windows XP. At the conclusion of the project the computer and all related model files will be turned over to the Office of Systems Planning. All algorithms and final data inputs will be put under the control of the DOT. The original version should not be modified, but scenario versions may be built upon the original version. The use of TransCAD as the software platform will permit an unlimited size for the number of zones and links in the model system Software Interface A computer interface will be custom-designed for the Iowa Statewide Model using TransCAD GISDK language. At a minimum the interface will identify the scenario, forecast year, and the data and program files to be used for the model runs. As part of the interface a file naming system will be established. In a later section of this report examples of typical screens from an existing statewide model interface are shown Base and Forecast Years The base model year will be 2005 and the goal will be to replicate traffic on an average weekday in Although the model will be developed in 2007, data such as traffic counts and socioeconomic data may not be available for 2006 or later years. The forecast year will be 2035, so that there will be a minimum 20 year forecast available at the completion of the model development. For interim forecast years (2010, 2015, etc.), the socioeconomic zonal data will be interpolated between the base (2005) and the forecast year (2035). The development of the 2005 and 2035 socioeconomic data and networks including interim years will be discussed in later sections. Interpolation may take the form of linear (straightline), or non-linear (polynomial or logarithmic) interpolation. The issues involved in how to interpolate are best addressed by the policy representatives of the Metropolitan Planning Organizations (MPOs) and the Regional Planning Affiliations (RPAs) involved in the process. The type of interpolation will be determined at the time of model scenario building, or before, and the selection of the type of interpolation will be at the discretion of the Iowa DOT with input from MPO and RPA planning staff. An important issue in the development of socioeconomic inputs is the preparation of a uniform 2005 base. The MPO and RPA participants currently have base years ranging from 2000 to Census household information is tied to year The employment data will be prepared by the Iowa Workforce Development Agency, and will be tied to their base year. One of the initial activities of the statewide traffic model process will be to bring each participant s socioeconomic data to year 2005 and to reconcile the results at the statewide level. In some cases there may be a decline in population or employment growth in specific areas, which will require understanding and acceptance from the representative of that group. 6

7 Since Iowa does not use a state demographer, a statewide control total for population and employment will need to be adopted from another data source. Agreement about the source and the final totals to be used in an allocation procedure will be among the initial tasks undertaken by the Iowa DOT. The Iowa DOT utilizes a regional economic model from Regional Economic Models, Inc, (REMI) that can be used to refine socioeconomic forecasts. Other privately developed county-level data can also be used to provide a base year control total. 2.2 General Model Outline The following are the general model components of the recommended Iowa Statewide Model for a passenger car sub-model and a truck sub-model (Phase II, Tier I). Each model component will require a Technical Memorandum that will document the assumptions made, methods used, issues discovered and resolved, and findings of that step (see Section ) Network Zone Structure External Networks and Stations Socioeconomic Information Trip Generation Trip Purposes Production Model Attraction Model Trip Balancing of Productions and Attractions Time of Day Special Generators Trip Distribution Friction Factors Doubly Constrained K Factors Mode Split Auto Occupancy Traffic Assignment Capacity Constrained Truck Model Truck Trip Types Small; Medium; Large Truck Trip Generation Rates Truck Trip Friction Factors Truck Assignment Pre-assignment and All-or-Nothing External-External and Internal-External Trip Tables Calibration and Validation Graphical User Interface See Figure 2 for the architecture process flow. 7

8 8

9 2.3 Model Structure It is recommended that the Iowa Statewide Model be national in scope for the highway network and zone system, with a decreasing level of detail as the distance from Iowa increases. The travel model team anticipates creating a buffer area of counties or smaller scale zones in the states that adjoin Iowa. This will allow tripmaking decisions for autos and trucks to be more accurately forecasted on major roadways in adjoining states that serve Iowa. Figure 3 shows a typical aggregation in scaling for the proposed zone structure. The Iowa DOT will take the lead on integrating the zone system needed by the statewide model and fed by the MPOs and RPAs within the state. The MPOs will require guidance on aggregating their TAZ system to reduce the number of zones. The RPAs will work with the Iowa DOT to develop an appropriate zone system for use in the statewide model. Figure 3: Typical TAZ Structure Geographic Units for Statewide Travel Models Highway Network It is recommended that the highway network for the Iowa Statewide Model be built from several data sources. The primary sources of network data will be Iowa s and the adjacent states road file information, the existing MPO model networks, the National Highway Planning Network from FHWA, and TransCAD or other GIS software s data bases. The inclusion of a highway link on the statewide model network will be a function of several factors. These factors are: The number of zones in the system and their size. Zones and the network must be compatible. Typically highway links will form the boundary of the zones. Thus the more zones, the more links will be in the network and the more inclusive the network will be. When creating the network, the highway links should not split the zones. Links splitting zones are extremely difficult to model especially when creating centroid connectors that reflect proper assignment loading. 9

10 Data availability. Links included in the network should be part of an existing database so that the network parameters are available. Even more important is that the network databases have identifiers so that their information may be electronically transferred to the model network using either the MS Link or Linear Referencing System (LRS) Anchor Section. MS Link is a unique identifier between data tables and the geographic road network. The DOT can link the statewide model with an existing road file or files by using the Iowa DOT MS Link system or the Linear Referencing System (LRS) Anchor Section. Functional class and traffic volume. The functional class should match those included in the speed and capacity tables. Extremely low volume roads (< 500 ADT) may not be of sufficient size to accurately model. However, they may be included if the area they are located in is expected to grow rapidly. To note, several roads on the Primary Highway System in Iowa have low traffic volumes. Special attention will be given to these roadway sections to insure their inclusion in the model network. Expected uses of the model. Roadways that are serving existing or future high intensity uses such as ethanol plants, anticipated sites for bypasses, crossroads at future interchanges, and other uses, will be included in the network. Consistency with the MPO model networks. The network will be consistent with the MPO model networks. Any link in the State Model network will also exist in the MPO model networks. However, all network elements in the MPO model may not necessarily be included in the statewide model. Networks outside of Iowa with significant external trips. Links that are anticipated to carry more than 5,000 ADT external trips will be included in the model network. Future networks as well as interim year networks should be built by constructing a Master Network, which is a single network that references future highway projects and allows the analyst to include or exclude the roadway in a given scenario. As an example, if an additional lane for I-80 is planned in 2015, the appropriate network links will exist in all scenarios but the number of lanes and resulting capacity will change between 2010 and See Table 1 below. The capacity improvement can, in effect, be turned on or off, as needed depending on when it is planned or programmed to enter the system. This temporal dimension built into the highway network facilitates the tracking of highway links over time and also allows all link records to be stored in a single physical file. The temporal variables can be named for the scenario or year in which the link is included and used to filter link records to form selected study scenarios. Launching of the Iowa Master Network will involve an initial outlay of resources, but the resulting network database will offer flexibility and power in using the statewide model. Transportation Improvement Plans (TIPs) and/or projects for each MPO and RPA should be reviewed to identify future transportation improvements. State projects will be tabulated as well. However, the plans and the TIPs will not be sufficient to identify all future transportation improvements and when they will be 10

11 built. Discussions with State, MPO and RPA planning staffs will be necessary to finalize the transportation improvements that will be added to the interim and future year networks. Table 1 shows two typical links and how they would be treated over three study years (2005, 2010 and 2015) in a simplified Master Network setup for a lane addition and for an entirely new minor arterial roadway addition. Table 1: Master Network Structure One-way Interstate Link - Lane Addition in 2015 Name Length Speed TIP2005 Cap05 Lanes05 TIP2010 Cap2010 Lanes2010 TIP2015 Cap2015 Lanes2015 I Yes 40,950 2 Yes 40,950 2 Yes 61,425 3 Minor Arterial Road Addition in 2015 Name Length Speed TIP2005 Cap05 Lanes05 TIP2010 Cap2010 Lanes2010 TIP2015 Cap2015 Lanes2015 Main St No 0 0 No 0 0 Yes 4, Number of Zones, Zone Size, and Centroids. The Iowa Statewide Model zone size will be relatively extensive. The purpose of an extensive zonal system is to support the inclusion of a complete network that will allow analysis of interchanges and potential lane additions to roads from the interstate scale to existing two-lane rural facilities. An extensive roadway system will also allow easier matching to the existing MPO networks. Additional information concerning zonal size will also be discussed in the zonal section of the architecture. Locating each zonal center of activity will identify the location of the centroids for each zone. The number of centroid connectors for each zone will be determined on a case-by-case basis and will depend on the highway system around each zone. Typical speeds for centroid connectors will be 20 to 25 miles per hour to reflect their local road character. Capacities for centroid connectors should be high enough (typically 10,000 vehicles per hour) so that they are not affected by a capacity restrained analysis. Digital aerials, land use or zoning information and GIS files for parks, waterways, and related sites can provide guidance in this task Functional Class & Traffic Volume. In rural areas, the statewide model network will include all roads functionally classified as rural major collectors and higher. In the urban areas, all roads functionally classified as minor arterials and above will be included. However, lower functionally classified roads will be included in the network if it is determined that they are connecting an important activity or it is anticipated that they may experience rapid growth in the traffic volume due to anticipated growth at their location. The network will be constructed so that roadways that carry at least 95% of the MPO external station traffic will be included in the statewide network. The statewide 11

12 model network will include nearby or adjacent urbanized areas outside of Iowa. These adjacent areas outside of Iowa will include a network density consistent with areas inside Iowa s state boundaries. All Interstates and freeways will be coded as two one-way links and all individual interchange movements will be fully coded. For the urbanized areas adjacent but outside of Iowa, the network density and zonal system will be similar to the areas within Iowa. The farther out one goes from the Iowa state boundary, the less dense the network will become Network Parameters. Network parameters that will be included for each link will include: Route Name Functional Class (Stratified by the Speed/Capacity Table) Number of Lanes in Each Direction Urban or Rural Identifier Traffic Counts (Vehicles and Trucks) Free Flow Speed Final Speed (Identified From Traffic Assignment) Link Capacity (Level of Service C) County and State Identifier Link Time Penalties Connectivity to intermodal terminals TIP Year with relevant variables to describe road changes. MS_Link (Iowa DOT) LRS Anchor Section ID (Iowa DOT) Speed and Capacity. The primary method for assigning the speed and capacity of each highway link will be through the development of speed and capacity tables. There will be two speed/capacity tables one for highway links in urban areas and one for rural areas. Urban areas will be defined by the boundaries established from each MPO. Table 2 shows the capacity lookup table that was developed for use in a recent statewide modeling study. Figures 9 and 10 in Section (Graphical User Interface) show representative speed and capacity lookup tables used for an update of another Statewide Model. Note that the base hourly lane capacity, obtained from the Highway Capacity manual, is the starting point for developing capacity. Other factors, such as area type, terrain, access control, and divided or undivided status, contribute to the model capacity of the highway links. The Iowa Statewide Model network link speeds will be developed using a similar approach, and linked, as is this capacity approach, to the needs of a statewide effort aimed at replicating daily truck and auto trips. 12

13 Table 2: Sample Capacity Lookup Table for Statewide Traffic Model Hourly Link Type Base Lane Area Type Terrain Access Control Divided Highway Capacity Urban Rural Level Rolling Full Partial None Divided Not Divided Interstate 2, Freeway 2, Expressway 1, Principal Arterial 1, Other Principal Arterial 1, Minor Arterial 1, Collector Major Collector Ramp 1,500 Source: Highway Capacity Manual Table 2-14, Capacity by Facility Type, and a Statewide Corridor Model from another state The link capacity has been defined at Level of Service C which can be explained as follows. Link capacities are used as part of capacity restrained and equilibrium assignments. (See also Section on traffic assignment below). Capacity restrained and equilibrium assignments are iterative assignment processes. For each iteration, the link speeds are modified based on the traffic assignment, the capacity, and the resulting volume-to-capacity ratio. Typically, the speeds are modified based on the Bureau of Public Roads (BPR) Speed\Volume Equation. A standard version of the equation is as follows: Adjusted Speed = Free Flow Speed / (Volume / Capacity) 4 When the V/C ratio is one, the denominator is 1.15, which would result in an adjusted speed of.85 of the free flow speed. The 85th percent of the free flow speed corresponds approximately to a Level of Service C. Therefore the standard BPR speed/volume curve uses a capacity as set for Level of Service C. It is recommended that the Iowa Statewide Transportation Model initially use the standard BPR Curve with the capacity at Level of Service C. Statewide models that typically do not have significant congestion will find that the standard BPR equation is sufficient to replicate existing traffic conditions. However, during model calibration the 0.15 coefficients and the exponent 4, as well as the Level of Service may be adjusted if necessary to match ground counts. During the model calibration step, consideration will be given to establishing a third area type (towns) for speed and capacity tables. Towns and cities will be classified as areas designated by the Census with a population between 2,500 and 50,000. Final speed and capacity relationships will be established as part of the model calibration effort. 13

14 Time Penalties. The use of time penalties will be incorporated into the network system. Time penalties may be especially useful at state border locations or crossings of major geographical barriers such as rivers. Typically there is a reluctance of travelers to cross geographical or state boundaries; especially for work trips and some non-essential home-based trips that cannot be explained by travel time. To reflect this reluctance, penalties are applied to the links crossing the political or geographic boundary. The actual value of the penalties will be established as part of the calibration and validation effort. The use of time penalties to capture intersection delay at signalized and unsignalized intersections, or serving all non-access controlled roadways is an issue that will be addressed in the statewide model development process. The questions that will drive the inclusion of such a delay component are: Is the scale of a statewide model appropriate for an input such as a time penalty for intersections? Is uniform data available to validate a delay module? Can a sketch approach to estimating delay at intersections be developed for Iowa that will add value to the statewide model effort? What are the final uses of the model? For example, if the Iowa DOT wants to test an add-lane project to U.S. 30 and gets an estimate of traffic diverted from I-80, will the statewide model support this effort? Are the modeled speeds on the two facilities as accurate as they need to be for diversion analysis? These issues will be addressed and a solution put forward at the time of initial data inventory and model development Zone System - The size of the zone system is typically based on two factors. The first factor is the level of aggregation of the socioeconomic data. The zonal system must be compatible with the Census geography since much of the zonal information will come from the Census. The second factor is based on the proposed uses for the statewide model. For example, if the model will be used to evaluate an intercity corridor, then the network and zonal system should be dense enough to consider corridor details such as secondary roads and interchange locations. A more dense zonal system and detailed network will also have the benefit of smoothing out the assignment results and better reflect actual traffic conditions Zonal Boundaries. The zonal boundaries will typically be consistent with Census geography, political boundaries, and where feasible, the highway network. These boundaries will include counties, towns, Census tracts, and Census block groups. All zones in the statewide model will be congruent with the zonal system in the urbanized areas both within Iowa and the urbanized areas adjacent to Iowa Number of Zones. The total number of zones is estimated to be from about 2,000 to approximately 3,000. For urbanized areas, the statewide zonal system will 14

15 use the existing system for each of the urbanized areas. However, the existing zones may be combined at a 2 to 1 or 3 to 1 ratio for the statewide model. Each State not adjacent to Iowa will be represented by from one to four zones depending upon proximity to Iowa, the State size, and their potential contribution to external-external and external-internal trips to Iowa. Bureau of Economic Analysis (BEA) zones will be utilized for out-state, non-adjacent zones. The farther out one goes from the Iowa state boundary, the less the number of zones, the larger their size, and the less the level of detail. The logical grouping of states into one or more zones will be explored for geographic areas farther away from Iowa (for example, the East and West coasts) Expansion - Dummy or placeholder zone numbers will be created in the model to allow for the future expansion of the zonal system. It is anticipated that a set of discussions will take place between the Iowa DOT and the MPOs and RPAs so that the statewide model TAZ system can be designed to capture future growth in the areas they represent. In summary, the final zonal structure will be determined during the development of the model. Development of the zonal structure will be a balance between meeting the needs of the identified model purpose, the boundaries of existing databases, the network density, and consistency with the zonal system of the MPO models External Networks and Stations There are two basic ways for statewide models to handle external trips; external stations versus a national network. For the Iowa Statewide Model combination of a national network and external stations is recommended. The national network will consist of the Interstate System with the potential inclusion of critical NHS connectors. As previously discussed, the areas adjacent to the state, in particular the urbanized areas, will have a zonal system and network more aggregated, but with adequate density and scale to the areas within Iowa so that traffic in Iowa can be captured accurately. By incorporating a national network system, national data bases such as the National Personal Transportation Survey, the American Community Survey, TranSearch, and the Freight Analysis Framework will have the capability of fitting within the Iowa Statewide Model and generating long distance trips through Iowa. Also, the need for conducting O-D studies at the State border will be reduced. Although a national network will be used, external stations will be established beyond the immediate Iowa state boundaries and the adjacent urbanized areas. The external stations will be the basis for defining external-external, external-internal, and internalexternal trips. An explanation of the external-external and external-internal procedure is in Section

16 2.3.4 Socioeconomic Data The zonal data that will be used as part of the trip generation modeling process will include, at minimum: Population Household Size Employment (Retail/Non-Retail) Auto Ownership The zonal data will be used as part of the trip generation process. Additional employment data may be needed as part of the truck model. The additional information will be identified as part of the development of the truck model Existing Data. The base year of the model will be Existing data will be obtained from the CTPP, existing urbanized models, and statewide databases, and adjusted to The Iowa DOT has leased REMI data, and most national consulting firms own national data sets of Woods and Poole for population and employment that can be utilized for developing population and employment projections. The best available data from REMI and Woods and Poole to develop statewide control totals should be used in conjunction with the MPO/RPA socioeconomic forecasts, to establish population and employment data for the Iowa Statewide Model. For statewide control totals, the WSA Team recommends starting with REMI and the MPO/RPA forecasts, then supplementing with Woods and Poole or some other national database. Most of the MPOs and RPAs already use Iowa Workforce Development data for their employment totals, and the 2000 Census for their population totals, and this approach can be reconciled with future REMI or Woods and Poole projections Forecasted Data. Forecasts of zonal data will be based on statewide control totals, trend analysis, and land use forecasts from the MPO models. Zonal data for zones outside of the State will be obtained either through national databases (REMI, Woods and Poole, etc.) or from the states. An initial forecast of socioeconomic data by zone will be made based on the information previously listed. After review by the Iowa DOT and the regional planning agencies, a meeting will be held with the DOT and local planning officials to resolve any differences between local forecasts versus the initial socioeconomic forecast for the statewide model. At the meeting, a review of the process for developing the forecast will be presented. Forecasting factors such as statewide population/employment control totals, national trends, historical Iowa population and employment trends, and population/employment ratios will be used to evaluate the socioeconomic forecast. The objective of the meeting will be to obtain a consensus as to the forecasted socioeconomic data that will be used as part of the statewide model. A second meeting may be necessary to reach consensus on the socioeconomic forecast. If consensus is not reached after the second meeting, the DOT will be the final arbiter of the control totals. 16

17 2.3.5 Trip Generation The basic structure of the trip generation models will be cross-classification for trip productions and regression equations for trip attractions Productions and Attractions. Two cross-classification models are recommended one for the urbanized areas and one for the rural areas. The crossclassification models will initially use rates similar to those shown below although the final values will be determined in Phase II. Note that Tables 3A and 3B show samples of home-based work trip rates stratified by auto ownership and household size for a Statewide Model. Table 3A: Home-Based Work Production Rates (Urban) 1 Person Household 2 Person Household 3 Person Household 4+ Person Household 0 Vehicles 1 Vehicle 2 Vehicles 3 Vehicles 4+ Vehicles Table 3B: Home-Based Work Production Rates (Rural) 1 Person Household 2 Person Household 3 Person Household 4+ Person Household 0 Vehicles 1 Vehicle 2 Vehicles 3 Vehicles 4+ Vehicles During the calibration of the model, modifications to the trip generation rate structure may be considered. For example, rather than using auto ownership, income stratified by low, medium, and high may be considered if they better replicate trip productions. The rates shown in Tables 3A and 3B were established using survey data collected in another state in 2001 and These rates are shown to demonstrate that a stratified approach (urban and rural) has worked successfully in an area similar to Iowa. The trip rates for each purpose are subject to modification during the model calibration. For attractions, the model will use attraction rate equations. The attraction equations will be based on a combination of the rates in NCHRP 365, the MPO models, ITE rates, and attraction rates from other statewide models. The selection of variables on which to base attraction rates is tied to the socioeconomic data that is both (1) available for the base year, and (2) can be forecast for each study year. Some employment variables, such as the number of manufacturing workers, are difficult to 17

18 predict. Population estimation can be straightforward, but the employment forecast by type of work establishment may be more demanding. As an example, during the development of a Statewide Corridor Model in a Midwestern state, employment estimates were made for seven work types: retail, office, educational, health, entertainment, manufacturing, and other and the attraction model then called upon all of them in attracting trips. In other statewide efforts three main types of employment have been used: service, retail, and non-retail. In each of these cases, the state DOTs and the MPO and other partners were prepared to contribute zonal level data at the detail needed or at a finer detail. It is possible to prepare a travel demand model with two types of employment: retail and non-retail. The overriding concern, however, is consistency among the many participants in the Iowa statewide model. The Iowa DOT has reviewed with the MPOs and the RPAs the types of employment currently in use in travel demand models; follow-on discussions will establish the most detailed level that is possible for all contributors. The Iowa Statewide Model attraction model will be developed using that data. Table 4 shows a sample attraction table from another Statewide Model, which references the three employment types mentioned above. Table 4: Attraction Rates Home Based Work Home Based Other Non- Home Based Service Employment Retail Employment Non-Retail Employment Dwelling Units Since attraction rates have traditionally been difficult to establish for statewide models, adjustments will need to be made to the model as part of the calibration process. It is important to note that in general the production trips will not match the attraction trips. Adjusting attractions to productions will be made for each trip purpose during the process of balancing the productions and attractions, except for Non-Home Based (NHB) trips, where productions are usually balanced to attractions. As is usually the case, it will be assumed that the production model will produce more accurate results and the attractions will be adjusted to match the productions. It is recommended that the TransCAD trip generation cross-class matrix procedure be incorporated as part of the modeling procedure. With trip generation a fully integrated procedure in the model stream, the model runs can be replicated whenever necessary as less manual processing is required to start a model run Trip Purposes. Trip purposes should be organized to contain trips that exhibit similar characteristics such as trip length frequency. For the initial Iowa Statewide Model the following trips purposes are recommended: 18

19 A. Home Based Work B. Home Based Other C. Non-Home Based D. Truck Trips (Medium and Heavy Trucks) E. External-External Trips (Auto Trips Passing Through State) F. External-External Truck Trips (Truck Trips Passing Through State) G. External-Internal/ Internal-External Auto Trips H. External-Internal/Internal-External Truck Trips I. Special Generators including Tourist Trips Peak-hour Volumes / Time of Day. In Executive Project Steering Committee (EPSC) discussions, it was determined that the Iowa Statewide Model will contain a peak-hour model obtained by developing and assigning peak-hour trip tables. The 24 hour trip tables will be converted to peak-hour trips based on hourly trip percentages by trip purpose. A ratio of peak-hour travel to daily travel for each trip purpose will be established using observed data from the Iowa MPOs, or other sources within Iowa. This information will be enhanced, where necessary, with national averages from NCHRP 365. Observed peak-hour traffic will be used to validate the peak models. The breakdown of the daily flows to hourly flows will make it easier to conduct future policy analysis scenarios, corridor studies, and to use the model results for design purposes. A post-processing application method to derive daily volumes for a corridor, sub area, etc. can be used as a back up to determine peak-hour volumes for links/roadway segments. There are several options to do this: The daily assigned volumes can be factored by an areawide average of all areawide peak-hour counts as a ratio of daily counts. You can also factor the daily volumes by a geographic area average of all peak-hour counts as a ratio of daily counts in that geographic area/sub-area (same for a corridor). The daily assigned volumes can be factored by an areawide average of peakhour Functional Class counts, by FC (so that you have derived peak-hour volumes based on factors for principal arterial, collector, etc.). And can do the same for a geographic area/sub-area (or corridor). NCHRP 365, Chapter 8, can be used if count data, or FC data is not available. Table 41 or Tables 44/45 can be used to derive peak-hour volumes Special Generators. Sites like national or state parks, airports, military bases, casinos (like the ones in Council Bluffs and Davenport), regional hospitals, colleges and universities, ethanol plants, intermodal centers, and tourist attractions may be 19

20 represented as special generators. The special generators will be treated as attractions and the preliminary rates will be established by using a combination of appropriate ITE rates, NCHRP 365 rates, and any approaches already in place for special generators in the MPO models. Site visits may also be made to establish trip attraction rates. Special generators rely on information other than household or employment to generate trips. For example, travel to and from an airport is dependent on the number of enplanements that take place on an average weekday at a given airport. Casino traffic can be estimated based on the number of slot machines that the facility contains. Many types of special generators are well studied with the result that trip rates and daily patterns can be readily estimated for them. There is, however, a small set of evolving uses specific to Iowa that will require attention during model development. One example of a new special generator that merits attention - and one that potentially affects truck traffic - is any facility related to ethanol. If in the next few years national policy ramps up bio-fuel production, Iowa will be a nexus of that activity. Rich soil and ample water supplies will drive continued high production of corn and other bio-fuels. Farm-to-market roads, storage and transfer facilities, and production and shipping centers related to ethanol, can be expected to grow rapidly in Iowa. A special generator such as an ethanol plant will need to be added to the list of special generators above. Likely field studies will be conducted on a set of representative ethanol-related sites in Iowa to determine the best way to estimate auto and truck traffic. The specific list of special generators, including the evolving ones, will be drawn up as part of the model development process. Selection of the generators will be based on the significance of their relative trip attractiveness. For example if a state park is a major attractor in a rural county then it would be included as a special generator Trip Distribution The traditional gravity model will be the technique used for distributing the trips developed by the trip generation process. Statewide models typically use multiple trip distribution models. Internal trips can be distributed using typical trip purpose-based parameters and validated to surveyed conditions, either from Iowa MPO surveys or from National Personal Travel Survey (NPTS) data. Trip distribution models can also be built for long distance auto and for truck trips using the gravity approach. Given the availability of calibration and survey data, the gravity approach provides the best tool for this model step. Trip distribution is a procedure that links the trip productions to the trip attractions for each zone pair. The resulting output of trip distribution is a trip table containing the number of trips between every zone combination in the modeling area. The standard Gravity Model is defined as follows: 20

21 Where: T ij = P i (A j F ij K ij / S A k F ik K ik ) T ij = the number of trips from zone i to zone j P i = the number of trip productions in zone i A j = the number of trip attractions in zone j F ij = the "friction factor" relating the spatial separation between zone i and zone j K ij = an optional trip distribution adjustment factor for interchanges between zone i and zone j Friction factors are the critical input to the Gravity model after trip ends are produced (See Section below). They are inversely related to the spatial separation of the zones - as the travel time increases, the friction factor decreases. Calibration of friction factors consists of finding the appropriate parameter values to replicate observed trip length frequency distributions. As reported in NCHRP 365, it has been found that the gamma function produces more realistic results than other forms of mathematical formulas such as power or exponential functions. The gamma formula is defined as follows: F ij = a x t ij b x e c * t ij Where: F ij = the friction factor between zones i and j a, b, & c = Model coefficients; t ij = the travel time between zones i and j e = the base of the natural logarithms In many ways, the trip distribution step is the most important in the travel demand process because it establishes how sensitive trips of differing purposes are to time and distance. This information needs to be estimated correctly so that when the time comes to assign the trips, the trip distribution step has prepared the correct number of trips for each trip length category for each purpose and that as a result, the traffic estimated by the model will fit the observed conditions. As an example, consider heavy combination trucks (semi-trucks), often observed making over-the-road or longer trips than do other commercial vehicles. The gravity model equations and resulting friction factors are established to produce a trip length frequency distribution that captures mostly long trips for heavy trucks. Figure 4 shows the results from a typical internal truck distribution model in which medium and heavy trucks are distributed. Note that in this sample date there are fewer heavy trucks than medium in the system, but they tend to travel a longer distance. 21

22 Figure 4: Trip Length Frequency Distribution for Medium and Heavy Trucks 4,500 4,000 3,500 3,000 # of Trips 2,500 2,000 1,500 1, Medium Heavy and over Minutes There are two types of gravity models; singly constrained versus doubly constrained. For the Iowa Statewide Model, the doubly constrained model is recommended. By using the doubly constrained model, both the productions and the attractions will be preserved in the distribution process Friction Factors. Friction factors are the basis for determining how distance, time, and cost of trips affect the distribution of average internal weekday trips. They are usually found as a declining function of time, distance, and cost or some combination of the three. There are a number of methods for establishing friction factors. These methods include establishing gamma functions which typically are natural log functions that have variables of travel time and distance. Friction factor tables may also be used and may be simply a representation of the gamma function equation. In the past for statewide models it has been difficult to find the right equation to represent the distribution of both short and long trips in the same equation. For the initial model development, two sets of friction factors should be used, one for the urban areas, and one for the rural areas. These will be developed for the Home Based Work, the Home Based Non-Work, and the Non-Home Based trips. This procedure is similar to the procedure applied in a number of Statewide Models in other states. Since rural trip making characteristics differ from urban trips, developing two sets of trip generation and distribution tables should better reflect the differences in their trip making characteristics. The Iowa DOT will develop initial friction factors from a combination of the existing Iowa MPO models, the Census Transportation Planning Package (CTPP), the NPTS, 22

23 NCHRP 365, and initial values from other statewide models. Additionally, the National Household Travel Survey (NHTS) add-on data will be available from Des Moines. These initial friction factors may be modified as part of the calibration process. The use of the CTPP for developing friction factors will be limited since the Census only contains information concerning work trips. However, the Census information will be useful for establishing the trip length frequency distribution and the appropriate friction factors for the work trip purpose. The CTPP will also be able to identify major commuter work trip patterns especially those involving long distance travel. Tolls may be included in the travel time impedance calculation if they have been converted into travel time. An intra-zonal travel time may also be calculated. The TransCAD software contains a time calculation module that uses an automated nearest neighbor approach to obtain intrazonal travel times K Factors. Initial use of K factors to adjust distribution tables should be limited. Typically, K factors are used to adjust trip distribution tables when the models do not accurately simulate existing ground counts across State boundaries or geographic barriers. K-factors should only be used as a last resort. Rather than apply K- factors, link time penalties should be applied to match the assignment with the ground counts at the state boundaries or major geographic barriers, such as wide bridges and rivers Mode Split Mode share, or mode split, is the third step in the traditional four-step modeling process. Mode share is calculated after the trip generation and distribution steps. During the mode share step, models are built to estimate the percentage of trips for each purpose made using a mode other than passenger vehicle. Typically MPO models contain a transit mode split model and may have a walk or bicycle modal estimation. However, outside of large urban areas, the percentage of persons who travel by public transportation is very small. Motorized trips made up 95-97% of the household trips generated in a neighboring state, according to a statewide survey conducted in That 3-5% included walking, school bus, bicycle and other non-automobile modes. Transit mode share estimation also involves building a transit network and preparing time and cost skims for transit tripmaking which will go into a mode split model, an effort that is not consistent with the needs of some states. Sketch approaches to the estimation of transit tripmaking are workable on a statewide level. As an example, the Census mode share for tripsto-work can be used to develop a percentage transit for Home Based Work trips by Census block group or tract. This percentage can be transferred to the statewide TAZs and used to adjust Home Based Work trips by TAZ after they are generated and distributed. In this neighboring state, which contains two major metropolitan areas, it was found that an application of this approach to Home Based Work trips translated to a 2.3% reduction in total work trips statewide. For other trip purposes, no data are available to identify transit shares. 23

24 The mode share area that is of direct application to a statewide travel model is the estimation and application of auto occupancy factors. For the initial Iowa statewide model the mode split function will convert passenger trips into vehicle trips. The conversion will be based on the application of auto occupancy rates to the person trip matrices by trip purpose. The rates will be based on Census data for work trips, NCHRP 365 for all trip purposes, and review of other statewide models. Local information from the urbanized areas will also be reviewed and utilized as appropriate. The auto occupancy rates that were used in another Statewide Model are shown in Table 5 below. Table 5: Typical Auto Occupancy Rates Trip Purpose Auto Occupancy Rate Home-Based Work Home-Based Other Non-Home Based 1.72 Long Distance Business 1.94 Long Distance Tourist 3.6 Long Distance Other 2.7 Since the initial model in Phase II (Phase II, Tier I) will only have two modes auto and truck logit functions to establish other mode trips such as rail, bus, and air will not be necessary. However, the model architecture has been designed so that in the future other modes such as rail may be added to the model as efficiently as possible Traffic Assignment There are basically three main types of traffic assignment procedures; all-or-nothing, stochastic, and capacity restrained (equilibrium assignments are a modified form of capacity restrained). Stochastic assignments scatter the assignment among all reasonable paths. Equilibrium assignments spread the assignments in an iterative process based on travel times modified by capacity restraint. For capacity restrained assignments, travel times are typically adjusted based on the Bureau of Public Roads speed/volume equation (see below). The most simplistic assignment procedure is the all-or-nothing method that simply assigns all trips to the shortest travel time path. The capacity restrained process (note that there are many variations of this process) is an iterative or incremental method that varies the assignment based on the links approaching their capacity. All-or-nothing assignments in past statewide model applications have resulted in what might be called lumpy assignments. Generally this has been due to large zone sizes and not including sufficient secondary road systems. In the past, FHWA has reported that some states that have tried other traffic assignment techniques have been disappointed with the results. The all-or-nothing assignment might work as well as the other techniques. 24

25 However, for the Iowa Statewide Model, initial development using a capacity restrained assignment procedure is recommended. As previously stated under Section , daily capacities and traffic flows will be estimated. This will allow a capacity restrained procedure to be used. Also by having a larger number of zones, the traffic assignment should give smoother results. A capacity restrained procedure is valuable because of the need to capture potential congestion both in the present and in the future on the Interstate System, especially in urbanized areas. The use of a capacity restrained procedure will provide more realistic assignments in the urbanized areas. Typical applications of a capacity restrained assignment involve at least 20 iterations. However, the number of iterations that will be used in the capacity restrained procedure will be determined during the calibration procedure. The WSA Team recommends starting with the traditional Bureau of Public Roads (BPR) speed/volume equation to determine the change in travel as congestion is approached. This equation relates link travel times as a function of the volume/capacity ratio according to the equation: T=t f [1+a (v/c) b] Where t = congested link travel time t f = link free-flow travel time v = link volume c = link capacity a,b = 0.15, 4.0 The alpha and beta shown above are those defined in the original BPR function. The coefficients and the exponents for the equation will be determined as part of the calibration procedure. In other statewide applications, it has been found that when the alpha and beta values are keyed to the functional classification, the assignment results can improve. Additionally, TransCAD Version 5.0 will be able to assign different trip tables using different assignment methods, a tool that may be of value for the Iowa Statewide Model. After the initial traffic assignment, as part the traffic assignment calibration, a review of the results should be conducted. If the review shows that the assignments are distorted, other assignment procedures should be tested. These procedures would include stochastic as well as equilibrium assignments. If the model results do not improve, then the capacity restrained assignment procedure will be the standard assignment procedure for the Iowa Statewide Model (except for trucks (see Section )) Truck Model The truck model will follow the procedures as outlined in FHWA s, Quick Response Freight Manual. Specific features in the model should include: 25

26 Truck Trip Types. The Iowa Statewide Model should develop separate internal-internal trip tables for medium and heavy trucks. Small trucks such as delivery trucks should be included in the passenger car model since they have similar characteristics as a typical non-home based trip. Definitions of medium and heavy trucks will be consistent with the Quick Response Freight Manual Truck Trip Generation Rates. The truck trip generation rates will be based on the equations found in the Quick Response Freight Manual with the trip rates based on the type of employment within each zone. External-external and external-internal truck trips will be based on national databases such as FHWA s Freight Analysis Framework O-D trip tables as well as any truck counts available in Iowa or from an adjacent state. TranSearch data will also be examined for use in developing truck trip generation rates Truck Trip Friction Factors. Separate truck trip friction factors will be used for medium and heavy trucks. The initial friction factors will be derived from the Quick Response Freight Manual Truck Traffic Assignment. Long distance truck trips have unique characteristics. Truckers tend to not vary from their designated routes. Studies conducted in two other states have shown that long distance truck movements typically do not alter their path even when there is congestion on the major routes. Therefore, for medium and heavy trucks an all-or-nothing assignment will be tested first in the development of the Iowa Statewide Model. If the truck assignment does not show a good fit to observed data, other assignment algorithms will be tested until the best approach is found. Truck trips will be pre-loaded (pre-assigned) prior to assigning passenger cars and smaller trucks. The medium and heavy truck volumes will be subtracted from the link capacities before the passenger car capacity restraint assignment process is run. This assignment will be compared to observed values for trucks and the generation, distribution or assignment adjusted until the model provides a reasonable fit to the observed truck traffic External-External and External-Internal Trips Separate external-external (E-E) and external-internal (E-I) automobile and truck trip tables will be developed. The trip tables will be based on national databases such as the NPTS, TranSearch, and the Freight Analysis Framework (FAF and FAF2). Auto and truck trips will be processed in separate steps so that scenario building will be available that treats E-E travel differently for autos and trucks. Considerable effort will be made to collect the most recent and detailed traffic counts available at the external stations that will lie well outside the Iowa state boundaries. As an example the I-35 external to the north will be in Minnesota while the I-35 external to the south will be in Missouri. Any O- D surveys available from the adjoining states will be incorporated into the Iowa Statewide Model external approach. I-80 offers additional challenges since Illinois traffic from I-80, I-88 and I-74 feed the Iowa I-80 portal from the east. 26

27 There are a number of ways to adapt national auto and truck information for use in a statewide model. Starting with a data inventory step is recommended, followed by a determination of the best fit of national data and a national zone system to the draft Iowa statewide TAZ geography. The goal of this effort will be to identify a national zone system that will serve both auto and truck traffic efforts and will use Iowa DOT resources efficiently. Statewide model efforts now typically include national networks and models (see Figure 5 below). Some of the rationale for including national networks and traffic is to capture the growth of container truck traffic on U.S. highways in the last two decades. FHWA estimates that it has doubled in the past fifteen years. Certain truck trade routes, such as I-80 through the Midwest, are among those with interstate truck traffic growing at the highest rate of all national routes Truck Trips. Development of truck trips at the external station will use an approach parallel to that used for autos. First the control total for trucks will be obtained from the external stations. Then the external-external and external-internal percentages will be estimated. Guidance on commodity flows in the form of national truck trips is available from the Freight Analysis Framework (FAF) 1. A national network and national O-D trips tables for commodities by trucks are available for processing and validating truck flows through and into Iowa. The first generation of FAF commodity flows, capturing 1998 conditions, delivered O-D tables at the state-to-state level only 48 zones representing the 48 contiguous states were generated. FAF2, capturing 2002 conditions, will provide flows for 117 districts within the U.S. plus a small set of international and port externals. FAF 2 will estimate future truck commodity flows for 2035 and is expected to be complete in spring of A sketch-level commodity sub-model that collects only the truck mode from the FAF data and prepares a FAF district-to-county-level allocation for trucks will be part of Phase II. These truck flows would be fit to the observed truck traffic at the Iowa external stations. The FAF truck movement information will be supplemented by interviewing major truck generator representatives of ethanol plants and intermodal centers. Once county-to-county truck trip tables have been developed, the movements will be broken down to the traffic analysis zone level. The conversion will be based on the population and employment characteristics of each zone. Forecasting future truck trips at the external stations will be developed using growth factors. The growth factors at each external station will be based on population and employment forecasts for each large district (FAF zone) outside of Iowa and each internal zone as well as their distance from the external stations as established by the model network. After forecasting the future external-external and external-internal truck trips at each external station, a gravity model will be used to forecast the future truck through trips and external-internal trip tables

28 Figure 5: Example of a National Zone System to Serve a Statewide Model Auto Trips. The data and available zone system for national passenger car traffic must be determined at the outset of the statewide effort. National data sources such as Woods and Poole, the 1995 American Travel Survey; Census 2000 Journeyto-Work (STP154, Interstate Work Trips) and other will be utilized. Woods & Poole socio-economic data would meet almost all the requirements with respect to county-based geography, national coverage, socio-economic variables, base year 2005 and future years up to 2035 in five-year increments. Historical estimates of 1990, 1995, and 2000 are also contained in the data set. This three-year trend data will be important because some of the base year trip tables may need to be produced based on growth factors between 1990 and 2000 or between 1995 and The 1995 American Travel Survey was sponsored by the Bureau of Transportation Statistics (BTS) and was designed to fill the data gaps in understanding origindestination passenger travel patterns at state and metropolitan levels. The survey reported trips that were 100 miles or more during 1995 for all purposes, all modes and all seasons throughout the U.S. If it is determined that this data set is the most recent source of information on long distance passenger trips, it will be utilized in the development of the national-to-iowa passenger vehicle traffic. NCHRP Report 365 also offers guidance on the estimation of external travel when survey information is not available. Although the NCHRP Report 365 external station procedure was designed for urbanized area models, the principles are universal and appropriate with some modifications to a statewide model. The following steps, 28

29 whether for a statewide or urbanized area model, are required for developing these trips: 1. Estimation of through trips at each external station 2. Distribution of through trips between stations 3. Estimation of external-internal vehicle productions and attractions; 4. Distribution of internal-external and external-internal trips between external stations and internal zones. The observed traffic for autos at the external station is the control total for the auto approach. For example, if I-35 shows 40,000 autos at an external station, the number of through autos plus the number of internal destined autos must sum to one-half of the count value. Typically, two-way travel is assumed for auto trips, that is, the number of autos entering and exiting a study area is expected to be equal, and the two directions contribute equally to fulfilling the E-I/I-E portion of the count. A relationship between the functional classification and the percent through trips can be developed on the basis that larger facilities such as Interstate roadways carry a higher percentage of long-distance travel than do U.S. or state highways. A county road that is operating as an external station may have 0% through traffic. The percentage of through trips at the external stations will be developed by borrowing external through trip percentages from states with similar geographic characteristics that have developed Statewide Models. The borrowed percentages will be checked by reviewing the 1995 American Travel Survey that reported the trips which were 100 miles or more in length. The results of the American Travel Survey will be expanded (in order to reflect 2005 conditions) by growth factors based on population increases, and compared with the results of using the borrowed through trip percentages. After an estimate is established for the number of through passenger trips at each external station, an external-to-external station trip matrix will be created by distributing through trips based on station volumes and travel times between the external stations. By subtracting the estimated through trips from the ground counts at each external station, the number of external-internal trips will be established. However, externalinternal trips will need to be subdivided by trip purpose. NCHRP Report 365 provides percentages for each trip purposed that could be applied to the E-I/I-E vehicle trips. However, the Report 365 percentages are for urbanized area models and external trip purpose percentages from other Statewide Models will be used to make a comparison and potentially used for adjusting the Report 365 values. It should also be noted that typically the external-external and external-internal models are built using vehicle productions and attractions instead of the person productions and attractions used for the internal zones. 29

30 A concept level snapshot is shown in Table 6 below. Note that the assumed through percentage is a value included here to convey the idea. Final through percentages will be developed as part of the validation step of the Iowa STM. Table 6: Typical External-Internal Production Approach 25% 50% 25% 100% External Station Direction Assumed E-E E-I ADT from Iowa Through % Volume Volume HBWV_P HBOV_P NHBV_P Total I-88 (west of IL 78) E 7% 11, ,230 2,558 5,115 2,558 10,230 I-80 (west of IL 78) E 7% 17,000 1,190 15,810 3,953 7,905 3,953 15,810 U.S. 75 (south of US 14) N 2% 1, , ,176 SH 5 (Missouri) S 0% To create external-external and external-internal trip tables, the remaining volumes that represent passenger trips at each of the external stations and the attractions for each of the internal zones within Iowa and the nearby adjacent areas will be input into the gravity model, which will be run using friction factors established for long distance trips (default values from other statewide models). The trips in the trip tables that go from external station to external station will represent the passenger through trips. If indicated by the initial validation results, the trip tables will be calibrated by using a synthetic O-D trip table procedure called matrix estimation (ODME) in TransCAD, Future auto trips at the external stations will be forecasted by establishing growth factors for each station. The growth factors will be based on forecasted population and employment growth of nationwide zonal system and the network travel time from each zone and the external stations. A gravity model will then be run to establish future external-external and external-internal automobile trip tables. The most efficient level of detail needed to incorporate external-external and external-internal auto traffic in the Iowa statewide effort will be determined by discussion with the Iowa DOT. Both data sources and the geography that the data resides in will drive the decision-making process. The critical issues are: (1) Data must contain both base year and future year information; (2) data must contain all continental U.S. states; (3) the summary geography must be at a usably fine detail level county if possible Calibration and Validation The Iowa Statewide Model will undergo both a calibration and validation effort. The calibration procedure will be applied to the individual steps of trip generation, distribution, and traffic assignment sequentially, so that each step is calibrated before proceeding onto the calibration phase of the next one. Specific calibration targets (for example, in Trip Generation, overall Attractions should be within 10% of Productions) will be determined in Phase II. 30

31 The calibration process will be critical in finalizing the: Link System Zonal System Trip Purposes Trip Generation Rates Centroid Connector Locations Friction Factors K Factors Time Penalties BPR Equation Parameters Assignment Iterations The validation process will compare the overall traffic assignments with the model simulation for the base year. Whereas calibration looks at the individual components of the planning process, the validation process will evaluate the overall capability of the model to replicate ground counts Validation Targets. The following recommended validation tabulations will be produced and analyzed: Observed and Modeled Traffic Volume Comparison This test totals the observed and the modeled traffic by volume ranges. The standard accepted value should be within 5% of the observed value. Percent Root Mean Square Error (PRMSE) This test, which measures the difference between model volumes and observed traffic counts, is where the variability of the traffic counts is most in evidence. It is typically presented with the links sorted by volume class. If the model fit were perfect, the root mean square error of each class would be equal to zero. The variability between observed and modeled volumes is usually highest in the lower volume links (under 2,000 vehicles per day) decreasing as the volume range increases. A value of less than 30 percent is desirable. Observed and Modeled Traffic Volume by Districts The observed and model traffic volume totals can be compared using the six Iowa DOT districts, or other geographic districts. Percent Deviation for Total Vehicles by Link This test is a visual presentation of the difference between the observed and modeled volumes. Each point represents a traffic count and the total vehicle volume that the model obtained through assignment. The maximum deviation curve is based on the concept that higher volume links in a statewide model should contain a lower level of error than lower volume links. For example, links with daily traffic of 10,000 vehicles may appear in a travel model with error of 35%. However, on the links with 31

32 daily traffic of 90,000 total vehicles, the error should be under 15%. An example of this type of plot is shown in Figure 6 below. Table 7 below shows an example of the tests on counted links that was performed as part of a Statewide Corridor Model validation in a midwestern state. A table similar to this one is recommended for the Iowa Statewide Model. Note that there are over 4,600 one-way count locations in the example and that the overall -3% difference is within 5%. Also, the RMSE values are within reasonable limits. Table 7: Traffic Count by Volume Class with % Root Mean Square Error Link Volume Group Number of Counted Links Observed Total Traffic Volume Model Total Traffic Volume Difference Estimated- Observed % Diff %RMSE , ,021-8,526-2% 60.81% , ,466-4,587-1% 51.93% , ,510 14,849 2% 38.21% ,286,711 2,291,487 4,776 0% 33.97% ,489,555 2,459,723-29,832-1% 32.38% ,642,106 4,578,492-63,614-1% 33.40% ,126,912 8,275, ,772 2% 31.49% ,705,058 3,276, ,228-12% 48.78% ,257,066 2,056, ,777-9% 42.51% ,395,914 1,304,479-91,435-7% 28.66% ,574,460 1,494,104-80,356-5% 19.85% , ,175-4,536-1% 4.30% ,445,375 1,424,277-21,098-1% 9.45% ,975,497 1,815, ,913-8% 29.14% > ,255,075 3,190,223-64,852-2% 18.40% Total ,001,701 35,012, ,357-3% 32.37% 32

33 Figure 6: Percent Deviation for Total Vehicle Calibration Links 100.0% 90.0% 80.0% 70.0% Percent Deviation 60.0% 50.0% 40.0% 30.0% 20.0% 10.0% Maximum Desirable 0.0% Traffic Counts (Thousands) n=4,497 As part of the final report, a detailed list of each count location, and both its observed and modeled values, will be provided Screenlines and Cutlines. Validation for statewide models includes comparison of observed vs. modeled traffic across a set of cutlines and screenlines. Cutlines and screenlines are imaginary lines that are defined as follows: Screenlines typically extend completely across the modeled area and go from cordon boundary to cordon boundary. For example, a river that passes completely through the area makes an excellent screenline. Screenlines are often associated with physical barriers such as rivers or railroads. Jurisdictional boundaries make good screenlines if they extend across the entire study area. Cutlines - extend across a corridor containing multiple facilities. They should be used to intercept travel along only one axis 2. Screenlines and cutlines will be established during the Phase II activities of the Iowa Statewide Model. The selection will be based on best practices, and will both use existing traffic count data, and indicate locations where traffic data needs to be collected. Figure 7 shows Iowa with the draft screenlines of I-80 and I Travel Model Improvement Program, USDOT, Model Validation and Reasonableness Manual,

34 Figure 7: Draft Screenlines for Iowa Statewide Model It is anticipated that a set of cutlines will be established from existing count locations. The EPSC, the six Iowa DOT Districts, and the MPOs will participate in selecting cutline locations and providing counts. The cutlines drafted and shown in Figure 8 answer the following types of questions and also ensure that validation covers both urban and rural parts of the state: In rural areas, is the model capturing representative volumes on U.S. Highways compared to observed data? At major river crossing boundaries, is the model showing that all classifications of roadways summed are close to observed traffic? Near major metropolitan areas, is traffic from one key direction captured with adequate accuracy by the traffic model? Note that Figure 8 provides a guide on the types and locations for cutlines but is for illustration purposes only. 34

35 Figure 8: Draft Set of Representative Cutlines in Iowa Finally, Vehicle Miles Traveled (VMT) can be calculated from the model run and compared to statewide total estimates of VMT. Alternatively, the modeled traffic can be used to calculate modeled VMT and these totals compared to counted VMT in the six Iowa DOT districts. The validation standards cannot be stricter than the quality of the data bases and traffic counts. The calibration and validation standards found in the TMIP/FHWA Model Validation & Reasonableness Checking Manual, NCHRP 365, NCHRP 255, and FHWA s Calibration & Adjustment of Systems Planning Models, will be used as the basis of the model evaluation. Due to the low quality of traffic counts and their inherent volatility, links with counts less than a 2,000 ADT will not be included in the validation process. If a large number of low volume roads exist in a portion of the State, then they will be combined to form cut lines for evaluation purposes. The location of these screenlines, as well as the number and location of the statewide cutlines will be finalized in Phase II. Validation outputs (comparisons and metrics for all of the tests listed above observed vs. model traffic) will be captured in a TransCAD GISDK program. It is anticipated that this program will be used in the validation effort to assist in finalizing 35

36 the model parameters for the base year. A similar GISDK program will be developed to compare two or more statewide scenarios Post-Processing of Model Volumes - Post-processing of modeled volumes is a fairly straightforward application, which is done to correct an over- or underassignment of model volumes. NCHRP 255 describes three methods for developing adjusted, future year volumes; the numerical difference, the percent difference, and a combination of the two. All involve taking the difference between the base year assigned volume and the base year traffic count (or AADT), and applying that difference to the future year assignment to achieve adjusted future year volumes Numerical Difference. For example, I-35 between US-30 and State Route 210 had a 2004 AADT of 35,100. If the Statewide Model base year assignment for that segment of I-35 were 42,300, the numerical difference would be -7,200 (2004 AADT minus the base year Statewide Model assignment). If the Statewide Model forecast a volume of 54,300 in 2030 for that segment of I-35, the adjusted 2030 volume would be 54,300 minus 7,200 (47,100). This approach usually results in smaller adjusted differences for future volumes, because it dampens the difference between the assignment and count (or AADT) Percentage Difference. Again, I-35 between US-30 and State Route 210 had a 2004 AADT of 35,100. If the Statewide Model base year assignment for that segment of I-35 was 42,300, the percentage difference would be -20.5% (2004 AADT minus the base year Statewide Model assignment, divided by the 2004 AADT). If the Statewide Model forecast a volume of 54,300 in 2030 for that segment of I-35, the adjusted 2030 volume would be 54,300 minus 20.5% (43,200). This approach usually results in larger adjusted differences for the future volumes Combination. A simple average of the results from the first two methods can be utilized to develop adjusted 2030 volumes. This approach combines the advantages of both the numerical and percentage methods. The average of 47,100 (the 2030 numeric difference) and 43,200 (the 2030 percentage difference) yields a combined 2030 adjusted forecast of 45, Graphical User Interface A critical element for making the Iowa Statewide Model user friendly is the development of a graphical user interface (GUI). The GUI will be designed so that individual scenarios, data files, and model steps may be mixed and matched within the Interface. The specific design of the Interface should be designed at the conclusion of the model development. Some background on why a batch program is important is discussed below. Running a transportation-planning model usually requires performing relevant procedures in a specific order. The procedures that are commonly used include crossclassification methods for trip production and attraction, trip balancing, shortest path computations, gravity model evaluation, and traffic assignment. There are several options to run a planning model in TransCAD: 36

37 The model can be run interactively by running relevant TransCAD procedures in the necessary order. All procedures would be run using the standard TransCAD interface. This approach however is unsuited for applications involving several files or for applications involving multiple runs of the planning model, since managing the files and the inputs/outputs becomes cumbersome. A GISDK script (a batch file) can be written to run some or all of the TransCAD procedures that would normally be invoked interactively. The analyst can then compile and execute this code to perform the required procedures. There are several advantages to creating a GISDK program that runs some or all of your various planning procedures: Running the model will be faster since it does not require the additional time necessary for interactive setup. Consistency of inputs and parameters across the various stages will be achieved with minimal user intervention. Model and file management will be easier, allowing multiple scenarios to be evaluated with minimal additional effort. The batch file may also be saved as a customized GUI in which the commands can be launched from a dialogue box. Both the batch file and the custom GUI treatment are recommended for the Iowa Statewide Model. In the figures below, sample screens are shown from another Statewide Model. Four representative modules of the final GUI are shown below: 1) Scenario Manager Setup 2) Speed Capacity Tables Input Speeds 3) Link Capacity Tables Input Capacities 4) Assignment Parameters Set up Assignment. The program is built so that the model inputs, such as speeds by functional class and terrain, can be automatically loaded within the model. However, if the analyst wants to test an alternative assumption on capacity, the test values can be input and the model run to get the test results. 37

38 Figure 9: GUI - Scenario Manager Figure 10: GUI Link Speed Inputs 38

39 Figure 11: GUI Capacity Inputs Figure 12: Assignment Parameters Model Documentation Travel Model Approach and Validation. Model documentation will be provided for the Iowa Statewide Model. The documentation process will be organized to provide a stand-alone draft final report at the end of each of the main model development tasks. These reports will include, at minimum: (1) TAZ and Network Structure, (2) Trip Generation, (3) Trip Distribution, (4) Truck Model, (5) Traffic Assignment, (6) Base Year Validation, and (7) Future Year Approach. The submitting and review process for these documents will take place on a rolling schedule (as each step is completed and validated) allowing for a measured effort by the Iowa DOT instead of all review-taking place at the end of the project. The sections will be assembled into the final travel model report Graphical User Interface (GUI) User Documentation. A separate documentation will be provided to guide users of the Iowa Statewide Model in the setup and running of the actual Caliper Corporation TransCAD user interface. This documentation will guide the user from the directory setup and naming conventions, through each input screen of the statewide model, to the conduct of the postprocessing routines. It will also include two or three sample runs that address typical sensitivity runs encountered with Statewide Models in general. One example is testing a future scenario with an increase in external-external truck traffic. 39