uidaho.eduThe Gravity ModelGlossaryTheory and ConceptsThe Gravity Modelhttp://www.webs1.uidaho.edu/niatt_labmanual/Chapters/traveldemandforecasting/theoryandconcepts/GravityModel.htm[10/6/2008 12:35:45 PM]The Gravity ModelThe gravity model is much like Newton's theory of gravity. Thegravity model assumes that the trips produced at an origin andattracted to a destination are directly proportional to the total tripproductions at the origin and the total attractions at the destination.The calibrating term or "friction factor" (F) represents thereluctance or impedance of persons to make trips of variousduration or distances. The general friction factor indicates that astravel times increase, travelers are increasingly less likely to maketrips of such lengths. Calibration of the gravity model involvesadjusting the friction factor.The socioeconomic adjustment factor is an adjustment factor forindividual trip interchanges. An important consideration indeveloping the gravity model is "balancing" productions andattractions. Balancing means that the total productions andattractions for a study area are equal.Standard form of gravity modelWhere:Tij = trips produced at I and attracted at jPi = total trip production at IAj = total trip attraction at jF ij = a calibration term for interchange ij, (friction factor) or traveltime factor ( F ij =C/tijn )C= calibration factor for the friction factorKij = a socioeconomic adjustment factor for interchange iji = origin zonen = number of zonesBefore the gravity model can be used for prediction of future traveldemand, it must be calibrated. Calibration is accomplished byadjusting the various factors within the gravity model until themodel can duplicate a known base year’s trip distribution. Forexample, if you knew the trip distribution for the current year, youwould adjust the gravity model so that it resulted in the same tripdistribution as was measured for the current year.The Gravity Modelhttp://www.webs1.uidaho.edu/niatt_labmanual/Chapters/traveldemandforecasting/theoryandconcepts/GravityModel.htm[10/6/2008 12:35:45 PM]Glossaryhttp://www.webs1.uidaho.edu/niatt_labmanual/Chapters/traveldemandforecasting/Glossary/index.htm[10/6/2008 12:35:46 PM] Use your browser's 'back' button to return directly to the previous page.Centroids-- Imaginary points within zones from which alldeparting trips are assumed to originate and at which all arrivingtrips are assumed to terminate.Cordon Line-- An imaginary line that denotes the boundary of thestudy area.Friction Factor-- A mathematical factor that is used to describethe effort that is required to travel between two points.Link-- An element of a transportation network that connects twonodes. A section of roadway or a bus route could be modeled as alink.Modal Choice Analysis-- The process used to estimate thenumber of travelers who will use each of the availabletransportation modes (train, car, bus) to reach their destination.Nodes-- Nodes are points at which links terminate. Links mayterminate at destinations or at intersections with other links.Routes-- Pathways through a network. Routes are composed oflinks and nodes.Study Area-- The region within which estimates of travel demandare desired.Trip-- The journey between one point and another.Trip Assignment Analysis-- The process used to estimate theroutes (for each mode) that will be used to travel from origin todestination. This process yields the total number of vehicles orpassengers that a particular route can expect to service.Trip Distribution Analysis-- The process used to determine thenumber of produced trips from each zone that will be attracted byeach of the remaining zones.Trip Generation Analysis-- A data collection and analysisprocess that is used to estimate the number of trips that each zonewill produce and attract.Urban Growth Boundary (UGB)-- An imaginary boundary thatencloses all of the land that is expected to be developed at someGlossaryhttp://www.webs1.uidaho.edu/niatt_labmanual/Chapters/traveldemandforecasting/Glossary/index.htm[10/6/2008 12:35:46 PM]point in the future.Utility Function-- A mathematical function that expresses theadvantages and disadvantages of a particular transportation mode.Zones-- Regions within the study area that contain homogenousland uses and can be described accurately by only a few variables.Theory and Conceptshttp://www.webs1.uidaho.edu/niatt_labmanual/Chapters/traveldemandforecasting/theoryandconcepts/index.htm[10/6/2008 12:35:47 PM] Travel Demand Forecasting can seem like a long and dauntingprocess when viewed as a whole. It is much easier to approachwhen broken into small steps. The discussions below should helpyou develop a basic understanding of the Travel DemandForecasting process.Overview of the TDF ProcessDescription of the Study AreaTrip Generation AnalysisCross-ClassificationMultiple Regression AnalysisExperience Based AnalysisTrip Distribution AnalysisThe Logit ModelThe Gravity ModelModal Choice AnalysisTrip Assignment