Chittenden County transportation modeling history dates back to the late 1960's. CCMPO has managed the regional transportation model for many years with ongoing updates and refinements reflecting new data and planning techniques. Model applications have included several regional transportation plans and corridor studies along with smaller area studies, including traffic impact studies for large development projects.
The modern history of CCMPO's basic model framework dates to 1993 when an AM and PM peak period model was introduced with two significant changes from previous Chittenden County models. First, mode choice was incorporated to allow explicit modeling of bus, rail, and non-motorized transportation. Second, the 1993 model included an integrated Land Use Allocation Module (LUAM) which used transportation accessibility in conjunction with other factors to estimate the location of new housing and employment in the county. The LUAM uses local zoning and environmental constraints to establish the maximum amount of housing and employment possible in a particular zone to constrain the amount of growth. The total amount of growth in housing and employment is an exogenous input to the model based on regional forecasts developed by the CCRPC. Another 1993 model refinement was the addition of feedback loops from assignment back to distribution and mode choice as well as from LUAM back to trip generation. The LUAM feedback provides the model structure to calculate "induced" land development as areas become more accessible with transportation projects included in future years of a model analysis.
The next significant model update was in 1998 and included base year land use reflecting 1998 conditions and a regional household travel survey to re-estimate parameters for the trip generation and trip distribution. Several new TAZs were created by subdividing zones in the rural areas to allow for more detailed analysis outside the urban core. This model received an award of excellence from the national Association of Metropolitan Planning Organizations (AMPO) in 1999.
In 2005, the AM and PM peak period models were converted to use the TransCAD software modeling platform. Previous model versions used a custom program developed by Resource Systems Group, Inc (RSG) referred to as the Integrated Transportation Model (ITM) for all steps except for assignment, which was handled with the T-Model program. The use of TransCAD introduced new capabilities for assignment using an algorithm designed to accomplish user equilibrium. RSG developed a specialized routine within TransCAD to explicitly include intersection delays in the link travel times input into the assignment.
The land use in the 2005 model was updated to 2000 base year conditions and data from the 1998 Household Survey, the 2000 Census Transportation Planning Package, and other sources were used in re-estimating model parameters. This update also included a license plate survey of vehicles traveling on Interstate 89 at the county boundaries to better estimate through trips also referred to as external to external or XX trips in the model.
The most recent 2008-2010 CCMPO model updates have followed a two phase process following a competitive consultant selection process. This work is now nearing completion - Phase I of the update was completed approximately one year ago and included updating the base year housing and employment data to 2005 and a transition to a daily model with hourly assignments. The previous 1993-2005 models only represented AM and PM peak hours. The transition to a daily model allows more detailed analysis of air quality issues associated with transportation while still retaining the capability to examine peak period travel demand. Phase II of this update included re-estimation of several model parameters following receipt of the 2009 National Household Travel Survey (NHTS) Vermont add-on data funded by CCMPO, VTrans, and the University of Vermont Transportation Research Center. Several additional refinements were made in support of CCMPO's long range transportation plan update currently underway.
As the travel demand forecast modeling practice has continued to progress, CCMPO staff have recognized that the agency's model system needs to be upgraded to keep pace with recent developments, and to provide the policy sensitivities required by decision-makers. A primary purpose of the CCMPO peer review was to develop a list of recommended improvements to the travel demand model and provide guidance on which would be of greatest value to CCMPO. Upon receiving the feedback from the peer review panel, CCMPO will begin to implement the prioritized improvements as funding and staff time allow.
