3.8 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.
The ‘Network’ application builds the highway network. The network build and preparation process in the NERPM-AB model system is performed using the Cube Voyager NETWORK and HIGHWAY programs.
- Application Name = Network
- Application Purpose = Builds the highway network from link, node data and speed/capacity lookup tables; creates zone-to-zone shortest paths
- Primary Input = Highway link and node data and attribute lookup tables
- Primary Output = Highway network (*.net) and highway shortest path skims (*.mat)
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 highway network module of the model includes a calculation of speeds and capacities via lookup tables that the model uses later in the trip distribution and vehicle assignment steps.
In the first pass of the model, after assignment, loaded networks are created which are then skimmed for creating 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) and three value of time (VOT) class combinations. 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
Non-motorized skims (time and distance) are also created for walk and bike modes.
The ‘Transit Network’ application builds and prepares the transit network for the model.
- Application Name = Transit Network
- Application Purpose = Builds the transit network from transit route and stop/station location data; creates zone-to-zone transit shortest paths
- Primary Input = Transit route, stop/station data and zone access connectors by submode
- Primary Output = Transit network and transit shortest path skims by sub-mode and access type (*.mat)
The transit model process in NERPM42 was originally developed from the 2005 Jacksonville Transportation Authority (JTA) regional travel model. The transit model relied on Voyager’s Public Transport (PT) program. 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.
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. Besides generation of auto connectors, important elements of the transit modeling included in the NERPM-AB model system (path-building and assignment) are conducted using Voyager’s PUBLIC TRANSPORT (PT) program. Auto connectors are based on a user routine (AUTOCON), which was developed for the FSUTMS standard transit model application. 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. Transit shortest path impedance values (transit skims) are slightly different than highway skims in that transit skims vary by sub-mode and the type of access/egress and include both in-vehicle and out-of-vehicle time contributions to the total transit travel time. A common transit path can involve being dropped off by car at a transit station, waiting for five minutes, riding transit to another station and then walking for five minutes to the final destination
Though there are a number of skims currently being out by the model, DaySim only needs the following skims for peak and off-peak periods:
- Walk bus
- Walk project mode
This is because DaySim automatically chooses the PNR or KNR station for a drive transit trip and it get the skims for the drive portion from highway skims files. Project mode is a placeholder for running alternatives analyses in which a proposed new mode is inserted and asserted.