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Vermont Agency of Transportation (VTrans) Statewide Travel Model Peer Review Report

3.0 The Vermont Statewide Model

This section of the report provides an overview of the Vermont statewide travel model, including a history of the model, a description of the model's components and functionality prior to the peer review, and a list of items to be accomplished through the peer review.

3.1 History of the Vermont Statewide Travel Model

The statewide model is an objective, analytical tool developed to assist in transportation policy making and infrastructure investment decisions, as well as aid in long-range planning. Efforts to develop the initial model for the state of Vermont began in the 1990s with processes run in the SAS Model Manager 2000 platform and the road network maintained in the TRANPLAN software format. The base-year 2000 model was improved in 2007 by transitioning to a GIS-based model framework using the CUBE software package. Further enhancements were then made to improve the correlation between model outputs and validation data.

The TRC, under contract with the Division of Policy, Planning, and Intermodal Development at VTrans, has hosted, improved, and applied the statewide model since 2008. In the fall of 2008, when the TRC transitioned to host the model, the following enhancements were made:

In addition to these enhancements, the TRC updated the statewide model to a 2009-2010 base year. This effort included updating employment and housing totals for TAZs and housing characteristics by town. Characteristics from roadway improvements made between 2000 and 2010 were incorporated into the highway network, as well as truck percentages for traffic counts by TAZ and cross-border traffic counts for external trips. Trip rates and regression equations for HBW, HBO, HBSHOP, and NHB trips were updated along with vehicle-occupancy rates and external trip-fractions by trip purpose. Trip-distribution impedance functions for the gravity model were also updated.

Being one of the smallest states in terms of population and home to only one MPO, Vermont's travel model is a useful tool for statewide travel estimation. The statewide model is designed for application in a variety of transportation planning studies and projects. It has been used for scenario planning in the 2009 Long Range Transportation Business Plan to evaluate traffic changes from proposed limited access roads in Morristown and Bennington and to evaluate the implications beyond the limits of the Chittenden County MPO's model.

3.2 Current Vermont Statewide Travel Model

The model, in its current state, can be used to aid in the following activities, as defined by VTrans staff:

The Base-Year 2009-2010 statewide model is comprised of 866 internal and 70 external Traffic Analysis Zones (TAZs). Residential information from both the 2006-2010 American Community Survey (ACS) and the 2010 US Census were used to input household distributions and characteristics in the state. Data from the 2009 Bureau of Economic Analysis (BEA) and the 2009 Vermont Department of Labor QCEW were used to disaggregate employment characteristics among the internal TAZs. Total employment figures used in the statewide model are categorized into five user-specific industry groups: Retail, Manufacturing, Non-manufacturing, Education, and Government.

The statewide model network includes Interstate, State Roads, US Routes, Urban Collectors, and some major rural collectors. The highway network consists of 5,250 miles of roadway, of which about 2,800 are on the state system. A total of 7.4 billion annual vehicle-miles of travel (VMT) were estimated for the 2010 base year network.

The statewide model employs a three-module process, including trip generation, trip distribution, and traffic assignment, to estimate travel demand and link flow throughout the state and across its borders using statewide demographic and employment data. The model estimates daily passenger-vehicle flow between TAZs for four trip types: home-based work (HBW), home-based other (HBO), home-based shopping (HBSHOP), and non-home based (NHB). Commercial truck (TRUCK) traffic is estimated directly from traffic counts as a fifth trip type. There is no currently existing freight model in the statewide model to disaggregate truck travel by medium- and heavy-commercial trucks or to investigate average daily commodity movement.

The model applies rates and coefficients derived primarily from 2009 National Household Travel Survey (NHTS) data. Previous NHTS sampling in Vermont had not been robust enough to use for an effective model update, as compared with sampling from larger states. With this in mind, VTrans purchased an add-on to the 2009 NHTS dataset, which resulted in approximately 1,700 Vermont households and over 3,800 individuals surveyed. The resulting per capita 2009 NHTS sample representation was among the best in the nation. The resulting tabulation of travel behaviors from this dataset forms the basis of the sub-modules used in the model.

The model estimates link flows from the TAZ-based population and employment information. The trip generation sub-module combines these TAZ-based characteristics with the town-based fractions of cross-classified number of household members and workers to calculate home-based trips produced by each internal TAZ. Trip attractions for all purposes and trip-productions for the NHB purpose are generated for each TAZ using purpose-specific regression equations, each of which utilizes a different set of employment and/or population characteristic. TRUCK productions and attractions are taken as a fraction of the NHB trip totals corresponding to the fraction of trucks in traffic counts in the TAZ. Total non-TRUCK external person trips are then subdivided by the other four trip types using an external trip-fractions table estimated from the NHTS data. The trip production output is held constant while attractions are adjusted by TAZ to create a balanced trip table.

The passenger trip distribution sub-module applies the balanced productions and attractions table, a matrix of free-flow travel times between TAZs (based on travel at five miles per hour over the speed limit, with terminal times from one to four minutes for origin and destination), and a set of impedance functions to generate a Production-Attraction matrix between all TAZs using the gravity model.

Because the statewide model is a daily model, all trips are assumed to return to their origin, rendering the final matrix diagonally symmetric. Therefore, the set of trip-specific matrices resulting from the gravity model application are transposed and averaged with their originals to create symmetrical matrices with trip totals identical to the balanced trip table.

Prior to the assignment sub-module, the symmetrical person-trip matrices resulting from the trip distribution step are converted to vehicle-trip matrices by applying vehicle-occupancy rates by trip type for internal and external travel (from the NHTS). The assignment sub-module employs a user-equilibrium optimization to distribute trips in the network, resulting in statewide vehicle flows and congested travel speeds by link.

Appendix D provides further detail regarding the development, functionality, and validation of the Vermont statewide model. Figure 1 provides an illustration of the TAZs and highway network in the model.

3.1 Vermont Agency of Transportation Goals for Peer Review

After the 2009-2010 base year update, VTrans is now considering a host of potential improvements. VTrans's continued commitment to travel modeling is evidenced by its partnership with the University of Vermont Transportation Research Center (TRC), the current developer and host of the Vermont statewide model, in improving the model's capabilities. The TRC and VTrans felt the TMIP peer review process would be useful to identify model enhancements.

VTrans sought a peer review geared toward statewide model specifications, design, and plan for future improvement. VTrans anticipated that the recommendations of the panel would be innovative, state-of-the-practice ideas and hoped to include the peer review recommended tasks in its work program to enhance model development and refinement. The panel was asked to outline any weaknesses of the existing model and assist in determining whether application of current or more advanced models could inform issues specifically faced by the State of Vermont, as deemed necessary by VTrans. It was anticipated that considerable discussion regarding the merits of moving towards more advanced sub-modules and determining which approaches are capable of providing the greatest benefits for VTrans would be the primary topics of the peer review sessions.

3.2 Previous Peer Reviews

The peer review convened in June and July of 2013 was the first peer review session conducted for VTrans regarding their statewide travel model.

The figure provides a map of the Vermont Statewide Model, which spans the entire state of Vermont and includes external markings along the borders of the state. The map highlights the major cities within Vermont. Montpelier, the State Capitol, is located just north of the center of the Vermont. Burlington, located in the Northwest portion of the State borders Lake Chaplain, while Rutland is located due south of the center of the State. The map illustrates several borders for the fourteen counties within the State, as well as borders for urban areas around the more major cities. The statewide model traffic analysis zones are also illustrated with red borders, while the Statewide model network is provided in blue. The state highway network includes major interstates including I-89 and I-93, as well as other major highways and arterials throughout the state. Mountains in the western portion of Vermont are illustrated in green.

Figure 1: TAZs and Network in the Vermont Statewide Model

Updated: 3/25/2014
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