3.13 Assignment

Highway

The PrT Assignment procedure step performs the final vehicle assignment by time period step. The PrT Assignment step is called in the Assignment and Skim Update group for each period in the full model procedure sequence.

Procedure Name = PrT Assignment

Procedure Purpose = Assigns vehicle demand onto the regional planning network

Primary Input = Vehicle trip tables and settings

Primary Output = Congested link volumes and congested link travel times (speeds)

The purpose of the trip assignment model is to load vehicle trips onto the representation of the transportation highway network. This process results in traffic estimates on individual links in the network that ultimately attempt to simulate general vehicular travel throughout the planning region.

The highway vehicle trips are loaded separately by occupancy class. The assignment classes are required in order to prohibit single-occupancy vehicles and truck from utilizing the high-occupancy vehicle only lanes on the highways and interstates. The external-external demand is also defined as a separate class as some links prohibits external-external demand. The assignment employs a user equilibrium method that is an iterative process to achieve a convergent solution where no travelers on the roadway network can improve travel-times by shifting routes. Throughout each of these iterations, Visum computes network-link flows, which incorporate link-capacity restraint effects and flow-dependent travel-times. At the end of assignment for each period, the congested link volumes are stored in user-defined fields for post-processing and analysis. Next, the loaded networks are skimmed to produce skim LOS variables to be used in the next global iteration of the model. At the end of the model run, validation reports are generated by comparing the modeled volumes to the observed traffic counts on network links.

Transit

Transit assignment is the process of loading the transit trips onto the transit paths. There are only two time periods (Peak and Off-peak) for transit assignment as opposed to four in highway assignment. Peak (PK) represents the combination of the AM peak and PM peak transit trips produced by Daysim, while Off-peak (OP) represents the combination of the midday and evening transit trips. All the transit trips that are assigned are generated from DaySim output. Only the transit portion of the transit trips are used to generate trip table for transit assignment. Transit assignment is performed at the end of the final feedback loop using the PuT Assignment procedure step. The PuT Assignment step is called in the Network Assignment: Transit PK and OP group.

Procedure Name = PuT Assignment Procedure Purpose = Assigns transit vehicle trip matrices onto the regional transit network; Assigns two (2) periods of transit demand onto the regional planning transit network, peak (PK) and off-peak (OP) which summed together represent the daily transit assignment results. Primary Input = Regional planning transit network and transit trip tables estimated from the DaySim activity-based model Primary Output = Daily (peak + off-peak) transit route-level ridership and boardings/alightings by stop and station

Boardings and alightings by route as well as station-level summaries are stored in their respective databases. The user can simply load the List layout files (.llax) in the /listings sub-directory to generate boarding summaries.