Chittenden County Transportation Modeling

Models are important tools in the transportation planning process. Through the use of models, CCRPC is able to estimate how the transportation system will perform in response to changes in the highways, the transit system, and/or land use in the region.

CCRPC has prepared a list of Frequently Asked Questions on the transportation model along with a glossary of frequently used modeling terms. For more information on models in general we encourage you to review some of the links below. Further information specific to the CCRPC’s model may be obtained from the RPC staff.

Frequently Asked Questions About the CCRPC Transportation Model

1. What is a transportation model?

Transportation models are simulations of the "real world" that can be used to show the impact of changes in an area on the transportation system (such as adding a new road or transit line, or increases in population or employment). Three important assumptions are part of any model used for transportation analysis:

Key characteristics of the system to be modeled can be described in terms of quantifiable variables (e.g., number of automobiles per household, household size, etc.).
There is a relationship between these variables and the behavior of individuals or of systems (e.g., the more automobiles per household, the greater the number of automobile trips per household). This relationship is most often expressed in mathematical terms.
This relationship is the same for a given population and remains constant over time.

The transportation model integrates long-term household choices, such as where we live and work, with short-term choices of daily activity like getting the kids to school and stopping at the grocery store.

2. Why does the CCRPC have a transportation model?

The CCRPC transportation model churns through loads of data on population, economic forecasts, and travel behavior to develop forecasts of future traffic conditions. These forecasts are one tool used by the CCRPC Board and planners to identify the kinds of transportation investments that will be most effective in reducing congestion, improving travel times, and meeting the goals of the Metropolitan Transportation Plan (MTP).

In addition, the model helps us to forecast future travel demand and prioritize transportation projects for the Metropolitan Transportation Plan (MTP). The model is also used to analyze the impact of regionally-significant land use developments on the transportation system.

3. What is the current status of the transportation model?

CCRPC retained Resource Systems Group (RSG) to update the existing transportation model in 2008-2011. The most recent update of the model prior to this effort was in 2005. There are a number of enhancements included in the current model:

The model base year is updated to the year 2005 with updated information and data on housing, employment, and travel times, including information from the 2008-2009 National Household Travel Survey Vermont Add-on;
The model now includes estimates for each hour of the day to examine daily travel characteristics. Previous versions of the model only included AM and PM peak hours.
The model now estimates truck freight trips in the morning and afternoon peak hours and accounts for the impact of truck trips on roadway congestion.

4. What is the 4-step modeling process?

For the past 40 years, transportation professionals have used a 4-step approach to model transportation demand. After establishing County land use, population, and employment trends, planners determine the following:

Trip generation: An estimate of the number of person trips generated at a particular location, and attracted to a particular location, based on the assumed relationship among socio-economic factors, land use characteristics, and the number of trips. Trip generation then leads to:
Trip distribution: An estimate of the number of person trips that originate in every zone in the study area, with destinations to every other zone. The result is a trip table that is used to determine:
Mode Split: An estimate of the number of trips predicted between each origin and destination, expressed by type of mode that is available for that trip. Mode split leads to:
Network assignment: An estimate of the number of trips via a particular mode that will take specific paths through a road or transit network. The end result, when all trips are assigned to a network, is an estimate of the total number of trips that will use each roadway in the network. When compared to the capacity of this link, planners can forecast the level of congestion that will occur at that location. This becomes the basis for assessing the performance of the transportation system.

5. What other tools are available to transportation planners?

Several tools are available for more detailed, or micro-level analysis. Software tools such as Synchro, Paramics, and VISSIM are gaining acceptance for detailed traffic studies of roadway corridors. The Highway Capacity Manual published by the Transportation Research Board includes methods for evaluating roadway operations at intersections (signalized and unsignalized) along with roadway segments in rural and urban areas. Geographic Information Systems (GIS) help planners understand the political, demographic, and environmental factors affecting transportation.

6. What assumptions are made when creating a transportation model?

Since we can’t know for certain what the future holds, the transportation model includes some assumptions about the future related to land use, population, and other factors to predict when and where we’ll travel. The following questions guide the assumptions:

How many jobs and people? CCRPC forecasts economic and demographic changes for the county to estimate how many people and jobs will be in the region in 20-30 years.
Where will jobs and people locate? The next step is to predict the location of these jobs and people. Different techniques are used depending on the nature of the analysis. Often, a land use model is used to distribute jobs and people into Transportation Analysis Zones (TAZs). This allocation is made using the most current information available from cities and counties about their projected housing and employment growth, local zoning, and environmental constraints.
How will people travel? CCRPC then forecasts how many trips people will take per household, where they will go, how they will get there (bus, car, carpool), and what route they’ll take.

7. What are the limitations of the transportation model?

Results of a model are only estimates – they cannot provide a definitive picture of what will happen in the future. Much like economic projections, transportation forecasts are greatly affected by the long-term economic health and attractiveness of the region, by population changes, and by the individual behavior of each person using the transportation system.

8. What are appropriate uses of the transportation model?

To estimate travel for long range planning (about 20 years);
To determine travel patterns and impacts for large, regionally-significant developments; and
To plan for future air quality impacts as a result of the transportation system.

9. What information can the model currently analyze?

The model can estimate the following:

Daily travel patterns;
AM and PM peak hour traffic congestion;
Travel modes (drive alone, share ride, bus, rail, walk/bike) likely to be chosen by individuals;
Summary outputs such as person trips, vehicle trips, transit trips, and congestion;

Sources:

Puget Sound Regional Council, Seattle, WA

http://www.psrc.org/data/models/trip-based-travel-model/
The Metropolitan Transportation Planning Process: Key Issues, A Briefing Notebook for Transportation Decisionmakers, Officials, and Staff , Transportation Planning Capacity Building Program, http://http://www.planning.dot.gov/documents/briefingbook/bbook.htm