3.9 Network Preparation & Skimming
For short distance trips, DaySim uses distance skims obtained from an all streets network. For all other trips, DaySim requires highway and transit skims obtained from the model highway and transit networks respectively. These skims are also needed for network assignment purpose. Initially, free flow highway skims are created and used. In subsequent iterations of the model, loaded/congested skims from the previous iteration are used.
Highway
The highway network in NERPM-AB v2.2 is maintained within the Visum version (.ver) file using the links and nodes network objects. The highway network was imported from the previous version of NERPM-AB. The highway network is processed for skimming using the built-in procedures in Visum (see Network Prep group in the full model procedure sequence). The initial freeflow highway skims are generated by the Calculate PrT Skim Matrix procedure step for each mode (see Network Skimming PrT group in the full model procedure sequence). The highway skim matrices are stored in the version file and exported in the required format for DaySim.
Highway networks represent the highway transportation system in the planning area represented by the regional travel model. A highway network includes a series of interconnected “links” connected to one another at “nodes” with each link and node containing a set of attributes relevant to simulating highway conditions. The links represent roadway segments and the nodes in most cases represent intersections (though not always). The most critical of these attributes in FSUTMS models are those pertaining to area type (AT), facility type (FT), and the number of lanes (NL). With these three user-specified link attributes, the network processing steps of the model include a calculation of speeds and capacities via lookup tables. The lookup table for speed and capacities is stored as a user-defined table (SPDCAP) in the Visum version file.
In the first pass of the model, after assignment, the loaded networks are skimmed for creating congested skim inputs for the next iteration. This results in the use of more realistic and consistent skims as the model run progresses.
Highway skims are generated for four time periods (AM, MD, PM, and NT). The skims are also created separately for drive alone (D1), shared ride 2-persons (S2), and shared ride 3+ persons (S3) modes. The skims values derived for these are time, cost (tolls), distance, and generalized cost (expressed in minutes and includes tolls).
Generalized Cost (in minutes) = Time + Toll/(Value of Time)/Occupancy + Toll service time
Non-motorized skims (time and distance) are also created for walk and bike modes.
Transit
Like highway network, the transit network is also maintained within the Visum version file using the Stops, Line, and Time Profile network objects. The transit model process in NERPM42 was originally developed from the 2005 Jacksonville Transportation Authority (JTA) regional travel model. The micro-coded network was carefully developed from the JTA model’s master network file. This network included micro-coded transit station information as well as all fixed guideway facilities and optional transit links. Although a single master network was implemented initially, it was later desired by the long-range plan update contributors to make the network scenario specific. The transit network from the previous version of NERPM-AB was imported in the PTV Visum version file for NERPM-AB v2.2.
Transit network modeling is an integral component of the NERPM-AB model system. The transit model in NERPM-AB has been established to accommodate walk and auto access modes for local bus, express bus, and rail transit service. Transit skims are generated by the Calculate PuT Skim Matrix procedure step (see Network Skimming PuT group in the full model procedure sequence). Like other FSUTMS models, NERPM-AB uses the AM designation for “peak” transit trips and MD (midday) for “off-peak” transit trips.
Transit paths are built from the transit network by calculating the generalized-cost impedance along every path between pairs of traffic analysis zones in order to identify the shortest path between all zones in terms of a composite distance, travel time, and cost measure.
DaySim only needs the following skims for peak and off-peak periods:
- Walk bus
This is because DaySim automatically chooses the PNR or KNR station for a drive transit trip and it gets the skims for the drive portion from highway skims files.
