Embedded Files
Input files specifying the highway network and associated parameters are located in the Inputs\Highway folder. See the Highway Inputs page for a list of parameter files and data dictionary of the highway link and note attributes.
Creating new highway facilities
Creating new highway facilities
Modelers should review the attributes listed in the highway link and node data dictionaries before modifying the highway network.
Facility type, number of lanes, and posted speed
Facility type, number of lanes, and posted speed
The most important attributes for highway facilities are the facility type category (FTC2), the number of lanes (NUM_LANES) and the speed limit (POSTSPD). FTC2 and NUM_LANES are used to determine the volume-delay function attributes and capacity. POSTSPD is used, along with the FTC2, to determine the free-flow speed
- The FTC2 must be set such that it is consistent with the categories defined for other roadways in the region - see the FTC2 table in the Highway Assignment page for details on the interactions of highway attributes and capacity determination.
- Note that NUM_LANES represents the number of uni-directional travel lanes. Turn lanes and merge lanes should not be included in the NUM_LANES.
- Auxiliary lanes (lanes added between interchanges) on Freeways, Expressways and Toll roads ARE included in the NUM_LANES, but the AUXLNDIST attribute must be set to the distance of the auxiliary lane to appropriately discount the impact on roadway capacity (see adjacent map of AUXLNDIST locations)
Consistency with the transit networks
Consistency with the transit networks
Transit lines in Cube are defined as highway node sequences. Changes to the highway network link connectivity or additional nodes may necessitate updates to the transit line file. The easiest way to assure consistency is to load the transit layer before editing the highway network. Cube will automatically update transit routes that utilize the highway network links that are modified. For example, splitting a highway link adds a new node in the transit route definitions that use that link.
Curvature and projection in the geodatabase
Curvature and projection in the geodatabase
The base SERPM 8 highway networks were created in a geodatabase and a shape file is included in the Inputs\GIS\ folder of the model catalog. As long as the highway nodes are consistent, the True Shape Display can be used to "snap" the link-based network to the projected shape file. The shapefile better represents curvature in the network and allows maps to be projected beneath the highway network for enhanced displays.
Highway network changes that require complex curvature should be made starting from the GeoDB and a new .NET and shapefile exported for application in SERPM 8.
Signal locations
Signal locations
SERPM 8 utilizes the existence of signals (recorded through the boolean highway node attribute SIGLOC) in the calculation of arterial capacities. Other signal attributes, such as cycle length, are not utilized in SERPM 8.
SERPM 8 network links with AUXLNDIST defined
Generate new seed network and skims
Generate new seed network and skims
The seed network and skims (specified under the Run Controls section of the SERPM 8 Scenario Manager) provide a representative starting point for SERPM 8 demand estimation. These inputs are only used in the initial speed feedback iteration. Subsequent speed feedback iterations use the updated loaded network.
More information on the use of seed skims in regional travel modeling is available in the Model Setup and Run page.
Generating new seed network and skims
Generating new seed network and skims
To generate a new seed network, the following steps are run:
- Generate a free-flow network [details below]
- Skim the free-flow network [details below]
- Run SERPM 8 with the free-flow network and skims as input and capture the converged network and skims as the new seed networks (recommended for new model year forecasts)
- Use the converged network and skims for scenario analysis
Generate a free-flow network detail
- Create a scenario with the desired highway network and land use inputs; save and select this scenario.
- Open the SERPM 8 Application Manager
- Navigate to Utilities -> Generate Seed Skims -> Free Flow Network
- Run the Network Program titled "Generate Free Flow Network"
Skim the free-flow network detail
- IMPORTANT: Update the 3 seed-skim scenario keys to be unique names (process will overwrite existing files). For example, if generating free-flow skims, the AM skim could be named: Inputs\Highway\seed_net_skmFF\X_AMHSKIMS.mat
- Update the Loaded network to see transit skims to point to the newly generated free flow network:
- Navigate to Utilities -> Generate Seed Skims -> Free Flow Network
- Run the application group by selecting the Run Application button and clicking "Run Current Group Only" (see figure)
When to update seed network and skims
When to update seed network and skims
Most changes to the highway network for project analysis would not necessitate an update to the seed network and skims. Unless the network update results in substantial changes in the travel time between many zones, it is sufficient to use the seed network and skims associated with the model year. An example network change that could require a new seed network would be a large scale highway expansion project.
Note that different seed networks will produce slightly different model results, especially on
Highway network checks
Highway network checks
Updates to the highway network can be checked by:
- a visual review (e.g. highlighting links by attribute to ensure correct coding);
- a connectivity review by generating free-flow skims (see above) and reviewing the skim matrix;
- an assignment review by verifying vehicles are being assigned to the new facilities; and
- a congested network review by comparing the congested skim values with the baseline network values.
SERPM 8 conducts a test load of the transit line during the initial network processing. The model will fail if the transit line file is improperly defined (e.g. node sequence is not found in the highway network).
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