Supplemental Trips

Soundcast primarily models daily activity patterns of people that live inside the region with DaySim. However, additional travel from adjacent areas and travel from people not included in the activity models must also be accounted for. A large amount of the additional travel not generated from DaySim is captured by the Truck Model. An additional set of models represents the additional trips traveling to, from, and through the region, which are referred to as supplemental trips. These include:

• external trips (including trips from outside the region to destinations within the region, vice versa, and trips passing through the region)
• trips to and from Seattle-Tacoma International Airport

The model flow of auto supplemental trips is a combination of work (commute) trips and non-work trips. Work trips are taken directly from observed LEHD data and applied as a trip table that is grown in proportion to employment. Work comprise 70% of external auto trips; the remainder are calculated as follows:

• the total number of non-work trips originating from external stations (zones) is calculated as the difference between observed counts and the total truck and work externals.
• trips are distributed according to 4k gravity model coefficients, assuming trip purpose of home-based shopping.
• trips to/from the airport are generated assuming the trip purpose of home-based other. These trips are distributed using the 4k coefficients; the 4k mode choice model is also applied to airport trips.
• work and non-work external trips are added together with airport trips to produce a trip table by mode and time of day.
• for each assignment period, this trip table is combined with daysim trips to assign all demand.