The existing Charleston Area Transportation Study (CHATS) model platform was first developed in 2003 as a 24-hour period model with a truck trip model based on the FHWA Quick Response Freight Manual. To support the model development, a regional household and external station origin-destination survey was also completed in 2003. In 2007, this model was enhanced to include a nested logit mode-choice model, transit assignment, time-of-day factors, and a travel time feedback loop. This effort was supported by a transit on-board survey and a travel time survey of major corridors.
The CHATS model is an aggregate, trip-based model implemented in TransCAD. It relies on aggregate traffic analysis zones that divide the region into small areas that form the basis for the model. Overall, the model includes 665 internal traffic analysis zones with 14 external stations.
The CHATS model network is a GIS-based network and includes both transit and highway links. Transit travel times are a function of the highway travel time based on facility type.
Like many trip-based models, the CHATS model segments trips into a number of different categories. Total trips are split into person, vehicle, and external trips, and person trips are further segmented by travel purpose. Table 3.1 shows the trip purposes used by the CHATS model.
Trip Type |
Segmentation |
|---|---|
Person Trips |
Home-based work |
Home-based school |
|
Home-based other |
|
Non-home-based |
|
Commercial Trips |
CV Auto |
Medium Truck |
|
Heavy Truck |
|
External Vehicle Trips |
Internal-External Auto |
Internal-External Truck |
|
External-External Auto |
|
External-External Truck |
As is the case in many trip-based models, the majority of the demand model components are dedicated to modeling person trips. Households are further stratified by certain household characteristics (including household size, number of workers, and vehicle ownership) for trip generation. The only segmentation that is used in later models is vehicle ownership, which is used by mode choice.
Special generators are used to create trips for the following purposes:
The trip distribution models are gravity type models and are done at the daily level. Distribution is done using a weighted highway time.
The mode choice model splits motorized trips into drive-alone, shared-ride2, shared-ride3, walk-transit, and drive-transit trips . Non-motorized trips are not segmented in the model.
Temporal factors are used to disaggregate trips by time of day. Three time of day periods are considered: AM peak (6 - 9 am), PM Peak (3 - 6 pm), and off-peak (9am-3pm and 6pm-6am). The AM peak period is factored down to hourly congestion with 50% of the volume in one hour for trip distribution and mode choice.
A truck model is included based on the Quick Response Freight Manual. This model was calibrated in 2003 and has not been updated.
The current model has a 2010 base year and 2035 forecast year. BCDCOG is beginning work to update the socio-economic forecasts to support a 2040 forecast year update for the next Long-Range Transportation Plan (LRTP).
The CHATS model is used primarily by BCDCOG to prepare the LRTP and all financially constrained projects are included in the model network. The model is used by consultants for specific traffic impact studies and has been included to some extent in the new statewide model.
The CHATS model outputs are used in ranking projects in the Transportation Improvement Program (TIP). Forty percent of the project score depends on the model output with 10 percentage points based on freight performance.
A recent application of the CHATS model was to produce an Environmental Impact Study for the I-526 extension.
Prior to the peer review meeting, BCDCOG identified several areas for which they wanted the peer review panel to comment and make recommendations. These items were presented as a series of questions grouped by effectiveness of the model, model framework, data/tool integration, and current model issues and were a focus during the meeting. The questions are detailed below:
Model Assessment
Model framework:
Data/tool Integration
