Travel Demand Model

Population Synthesis

The Population Synthesis is developed through a statistical model that divides the number of households by income level, household size, and vehicle ownership. Secondly, the model utilizes statistical procedures to expand the data to match the totals based on individual data records and sample weights from household surveys.

Trip Generation

It determines how many trips will be produced in each model zone, as well as how many people will be attracted to each of the trip’s purposes. These characteristics include employment, vehicle ownership, and household size as they estimate the number of trips a person may make in one day.

Trip Distribution

Once the model recognizes the total number of trips beginning and ending in every subarea of the Washington metropolitan area, it must decide which pairs of TAZs those trips will move between (for each trip purpose). For example, if one hundred shopping trips begin at a TAZ, the model uses travel times estimated from the network file to find nearby zones where shopping trips are attracted and the TAZs match. Once every trip is matched, an origin-destination table is developed that shows the number of trips between each combination of TAZs. 

Mode Choice

Once people know where they are going, they must choose how to get there. If they own a car, they could drive, carpool, walk to transit, or park and ride. Based on the number of vehicles available in each household and TAZ attributes (land use, transit accessibility), the trips for each origin-destination pair are split among the various modes of travel. IS used A multinomial nested logit choice model that distinguishes between the three highway modes (SOV, HOV2, and HOV3+) and the four transit modes of access (Walk, Bike, Park-n-Ride, and Kiss-n-Ride) 

Trip Assignment

Route assignments in the model are determined by the number of vehicles traveling on the roadway network and where they are trying to go. Vehicles then try to find the shortest travel time to their destination. Each trip follows more or less the same route in successive runs. Based on the number of vehicles using each road segment and the capacity characteristics of that segment, the new travel speed for each road is updated.