Regional Travel Demand Model
The Seacoast Regional Travel Demand Model (“model”) is a set of technical tools developed by Resource Systems Group for the Rockingham Planning Commission (RPC) and Strafford Regional Planning Commission (SRPC) to assist in the long and short range planning, prioritization, and coordination of transportation projects in the Portsmouth-Kittery and Dover-Rochester metropolitan areas, as well as portions of the Boston Urbanized Area, in New Hampshire.
The Seacoast Model is a four step Regional Travel Demand Model with an additional module to summarize results and assist with tabulating emissions totals. The model uses TransCad as a software platform and includes a set of macros and routines prepared by Resource Systems Group to tailor the process to the region. Land use inputs are allocated for each of the traffic analysis zone (TAZ) in two housing categories (single family and multifamily) and six employment categories (low commercial, high commercial, retail, industrial, institutional and hotel/motel). The total amount of employment and housing for the region is derived from the outputs of independently developed growth projections to set an overall growth cap and is manually assigned to TAZs which allows for multiple land use scenarios when desired. The land use distribution information is utilized as a basis for the 4-Step Transportation model to generate traffic volumes and patterns. Known land use restrictions (zoning and environmental) are accounted for, and the new land use can be placed into specific zones based on existing development patterns and trends, or on future desired patterns and trends.
The Traffic Analysis Zone or TAZ is the basic unit of geography within the model and serves as a basis for generating traffic. The Regional Travel Demand Model has 533 internal zones and 23 external zones. Within the model, zones vary in size based on the size and population of the community.
The model was most recently updated in 2012 and utilizes 2010 as a base year for analysis and as a starting point for future year analyses. The 2012 update is based on updated housing and employment estimates for each community in the region and also includes a revised highway network and a new transit network.
Model Calibration
Estimation, Validation, and calibration are all performed during all steps of the Model: land use allocation, trip generation, trip distribution, mode split, and assignment. For the purposes of the FHWA however, calibration is typically the measure of how well the model assigned traffic volumes match with ground counts gathered in the region. This is measured against FHWA guidelines utilizing 3 statistics:
- Correlation coefficient – the overall statistical fit (r2)
- Percent Error Region-wide – The differences of the sum of all model assigned and all ground count volumes
- Sum of Differences by Functional Class – The same percent error as Region-wide, but calculated individually for each class of roadway.
The model is calibrated to the Design Hour Volume (last Friday in August) based on ground traffic counts gathered from 2009 to 2011, and was last calibrated and validated during 2012. Comparison of each model to the FHWA Guidelines is shown in the table below:
Table 3: RPC/SRPC Model Calibration Statistics
Calibration Statistics | FHWA Guideline | AM Model | PM Model | Off-Peak Model |
Correlation Coefficient (r2) | ≥ 0.88 | 0.92 | 0.93 | 0.92 |
Percent Error Region-wide | ±5% | -1.5% | -1.2% | -4.7% |
Sum of Differences by Functional Class | ||||
Freeways | ±7% | -5.4% | -1.4% | 1.5% |
Principal Arterials | ±10% | 4.4% | 0.1% | 3.7% |
Minor Arterials | ±15% | 2.6% | 6.7% | -11.9% |
Collectors | ±25% | -8.0% | -12.7% | -18.0% |
Emissions Calculation
Emissions modeling is a requirement of the Clean Air Act in regions in nonattainment for certain human-health impacting compounds, such as ozone and fine particulate matter. Running and non-running mobile source emission inventories are developed with the help of a travel-demand model. Vehicle miles traveled (VMT) estimates from a travel demand model are combined with emission factors from an air quality model, in this case EPA’s MOVES, to calculate the total transportation sector contribution of pollutants into the atmosphere. The RPC and SRPC regions (and the rest of New Hampshire) are currently in attainment for both ozone and fine particulates which means there is no requirement do demonstrate transportation conformity at this time. That being said, the maintenance and continued improvement of the regional travel demand model is important as stronger emissions standards may require the transportation conformity process in the future and emissions calculation remains a component of the Travel Demand Model that is used for the Long Range Transportation Plan and other efforts.
Motor vehicle emissions calculation is a post-process routine that utilizes the volumes and travel speeds output from the model. The model estimates VMTs for each direction on each link for each class of vehicle. This information is then provided to NHDES Air Resources Division to develop an emissions estimate utilizing the EPA MOVES simulator.
