Four Step Model Explained: Trip Generation
Trip generation is likely one of the easiest parts of the four step process. Normally, the most difficult part of dealing with trip generation is getting the input socioeconomic (population and employment) data correct. This post explains how trip generation is calculated in the model…
The socioeconomic data for most models includes the following attributes for every zone in the model area:
- Population
- Households or Dwelling Units
- Dwelling Unit Occupancy
- Auto Ownership Information
- Hotel Information
- School Enrollment
- Employment Information (number, type)
- Special Cases (airports, amusement parks, etc)
Many models have several fields that describe these basic attributes. In some, auto occupancy may be defined as a percentage of households with 0 autos, a percentage with 1 auto, and a percentage with 2 or more autos. In others, there may be percentages based on the number of vehicles compared to the number of workers in a household.
The simple part of trip generation models is that once the socioeconomic data is in place and trip production factors are in place, trip generation is simply applying factors to determine the number of trips generated by and attracted to each use. These factors are determined by a trip survey.
Trip generation is normally done with a cross-classification model. This is where multiple factors are used to determine the rate of trips per unit (household, employee, etc).
This spreadsheet is an example of trip generation calculations. It is a cross-classification model based on household size and auto ownership. For each purpose, the dwelling units with each criteria of auto ownership is is calculated and then applied to the trip rate in the ‘Production Table’ tab. For attractions, I simply applied a rate per employee to the employees by sector. The SEData sheet indicates the inputs and outputs.
The final output is a set of productions by trip purpose and a set of attractions. These are intentionally close with attractions higher than productions. The reason why will be discussed in the Trip Distribution post.
A trip purpose is basically a reason for travel. Most models include home-based work (which are any trip going to or coming from the home and work), home-based school, home-based other (trips for purposes other than school and work), and non-home-based (trips that do not go to or from the home). Some models may include home-based shopping or home-based social-recreation separately, and some combine it into home-based other.
Tags: trip generation
