Network Changes
ODOT delivered a Visum version file containing links, nodes, zone centroids, connectors, and transit routes from Emme. No geographic edits were made to links/nodes. The following attributes and settings were modified:
The network had already been projected in the OGIC coordinate system. The Visum projection setting was updated to reflect this.
The TSysSet field in Visum specifies which, if any, transportation modes are allowed to use a particular link. Most links allow a (auto), b (bus), Walk, and Bike modes. Some links provide connectivity to transit nodes, which is represented by the w (walk to transit) mode. The a, b, and w modes are preserved from Emme while the Walk and Bike modes allow for non-motorized skimming in Visum. Walk is allowed regardless of one-way vehicle operation on non-freeway links. Similarly, Bike is allowed on most auto links in the allowed direction, but not on freeway links.
Modified Volume Delay Functions (each link type uses one VDF) to match Emme.
Auto capacities (CapPrT) are calculated before assignment in a procedure sequence, which converts the Emme per-lane capacity to directional capacity for Visum.
t0forPuT is a procedure-calculated attribute for transit speed. See Transit Skim/Assignment Settings.
TCURFORPUT is not used.
In Visum, all links have a complementary link in the opposite direction, regardless of whether that link is needed in the network. Transport systems should be removed from these links if necessary. There are 410 of these links added in the Visum network:
- 307 walk to transit links, which allow walking connections in downtown transit hubs
- 38 freeway links, which are wrong-way I-5 links
- 1 minor arterial link, which is the wrong-way in a slip lane
- 64 ramp links, which are wrong-way I-5 ramps
The auto mode has been removed where travel in the added direction is not allowed. These links should not impact auto assignment. However, walk links added in the opposite direction some may allow some faster walk paths.
U-Turns (type 4) were prohibited in the network by removing the a mode (Auto) from allowed transport systems for these turns. Additionally, wrong-way turns were disabled on freeway ramps and to one-way links.
Connector free-flow times (t0) were updated using connector speeds from the Emme network. This allows for custom speeds on connectors rather than a global speed setting for connectors. This is described on the Auto Network page.
EMME_DWELL is an attribute transferred from Emme. Although dwell times are supposed to be included in transit run times, these add additional time. Emme dwell is stored in minutes, but was applied in seconds because this best aligned with Emme skims.
EMME_HEADWAY is an additional attribute in each line's time profile, which defines the headway.
EMME_TTF_FACTOR is another attribute, used in setting transit run times.
These settings are held in the skimming and assignment procedure sequences.
Transit skimming and assignment uses Visum's Headway method. Each line's time profile has an EMME_HEADWAY attribute that stores the headway length in minutes. Emme assumes an average wait time of half the headway, while Visum applies one of a set of user strategies to minimize wait time. This accounts for actual behavior of people using transit, such as knowing scheduled times and coordinating transfers. The amount of information available to riders is a setting for transit skimming and assignment.
We tried multiple combinations of settings and passenger strategies, including the "constant headways" option. "None - Optimal strategies" matched Emme skim results best. This means that passengers do not have information about their expected wait and headways are exponentially distributed rather than being constant, according to the Visum 2020 Manual. A property of this is "memorylessness", meaning in this case that the expected wait time is not dependent on the last arrival.
The network in Emme relied on transit time factors to calculate transit run times on links. The transit run time is meant to include dwell times, per the JEMnR documentation, by increasing time and decreasing speed for the transit mode:
Because the dwell time at stops and layover times are reflected in the average speed, the dwt and lay attributes are set to the minimum values of .01 and 0, respectively.
JEMnR User's Guide (p. 55)
The Transit Time Factors are described in the JEMnR Model User's Guide:
| Attribute | Description | Value | Notes |
|---|---|---|---|
| ttf | Transit time function number on links and turns | 0 – Use default speed 1 – 1.11*Auto time 2 – 1.33*Auto time |
Specify the transit travel time function to reflect the level of roadway congestion on transit travel times. Usually it is set to 1 – for Interstate and State Highways 2 – for all other facility types Set to zero if default speed is to be used. |
The transit run times with TTF applied and the dwell times are used to update time profiles in the Set Run and Dwell Times procedure.