Trip Generation
Trip Generation for the SCOG is handled in a spreadsheet separate from Trip Distribution and Assignment, which are performed in Visum. Excel spreadsheets for 2018 and 2045 are located in the Trip Generation folder.
The Trip Generation spreadsheet is organized into the following tabs:
- Inputs contains trip rates, parameters, and auto occupancy factors
- Trip Rates contains the trip rates formatted for the Calculated Trips tab, but these are unadjusted
- Zonal Adjustment contains the adjustment parameters from Inputs or can be manually changed
- Calculated Trips contains the total trips (households × trip rates) for each purpose, multiplied by the related Zonal Adjustment
- Skagit Co Balance performs P-A balancing on the calculated trips, such that the total Productions are equal to the total Attractions. Non-Home-based productions are set equal to the Non-Home-based attractions for each TAZ, because NHB trips are not produced in the household TAZ.
- Skagit Co Export is formatted for copying directly into Visum
Households are grouped into bins by number of persons (1, 2, 3, 4+) and number of workers (0, 1, 2, 3+). Households with 3 or 4+ persons and zero workers or 3+ workers (marked with an asterisk (*) below) are grouped together because there was a lack of representative households for these classifications in the survey. Rates for these groups are calculated as survey-weighted person-trips divided by the number of weighted households.
| W0 | W1 | W2 | W3+ | |
|---|---|---|---|---|
| P1 | ||||
| P2 | ||||
| P3 | * | * | ||
| P4+ | * | * |
Household trip rates were calculated for each household cross-classification group using the Whatcom Regional Transportation Study (2018). As of early 2021, a Skagit County household travel survey was delayed due to the COVID-19 pandemic. The most recent household travel survey available for Skagit County at the time of development was the 2008 North Sound Travel Survey (Whatcom, Skagit, and Island counties).
Trip rates calculated from the Whatcom Regional Transportation Study (unadjusted)
The Survey folder contains instructions and R code for processing a household travel survey using the same format as the 2018 Whatcom survey conducted by RSG.
- Auto Mode Share is the percent of trips taken in autos, defined as 100% minus the shares of walk, bike, transit, and ferry trips
- Employment-based Adjustment Factor adjusts the number of Home-based Work trips to match total regional employment
- NHB Trip Adjustment adjusts the number of Non-Home-based trips to account for commercial vehicle travel not captured in the household travel survey
- HBO Trip Adjustment is a factor applied to Home-based Other trips
Auto Occupancy Factors are used to convert person-trips to vehicle trips and were calculated for each household size bin and trip purpose using data from the 2008 North Sound Travel Survey for Skagit County households. Auto occupancy is defined as the average number of people traveling on trips in household vehicles.
Auto Occupancy Factors calculated from the North Sound Travel Survey
The Zonal Adjustment tab applies the overall parameters shown above and can be used to manually adjust trip generation per zone.
Trip Rates (Productions and Attractions) adjusted for Auto Occupancy
The SCOG Model generates trips for 3 purposes:
- Home-based Work (HBW) trips are between home and work
- Home-based Other (HBO) trips have one end at home and do not involve the work location
- Non-Home-based (NHB) trips are between two locations that are neither home nor work
Each purpose is later assigned in Visum as a separate demand segment.
External trips (where any trip end is outside of internal model TAZs) are not generated in the spreadsheet. External trip matrices were created by disaggregating passively-collected smartphone location data from large districts to model TAZs. The resulting matrix was scaled to match traffic counts at the model external stations.
See the Running the Model page for instructions on importing trip generation outputs to Visum.