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The Four Step Model


The history of demand modeling for person travel has been dominated by the modeling approach that  has  come  to  be  referred  to  as  the  four  step  model  (FSM)  (see  Chapter  2).    Travel,  always  viewed  in  theory  as  derived  from  the  demand  for  activity  participation,  in  practice  has  been  modeled  with  trip-based  rather  than  activity-based  methods  (as  presented  in  Chapter  4).    Trip  origin-destination (O-D) rather than activity surveys form the principle database.  The influence of activity characteristics decreases, and that of trip characteristics increases, as the conventional forecasting sequence proceeds.  The application of this  modeling  approach  is  near  universal,  as  in large measure are its criticisms (these inadequacies are well documented, e.g., by McNally and Recker (1986)).  The current FSM might best be viewed in two stages.  In the first stage, various characteristics  of  the  traveler  and  the  land  use  -  activity  system  (and  to  a  varying  degree,  the  transportation  system)  are  "evaluated,  calibrated,  and  validated"  to  produce  a  non-equilibrated  measure  of  travel  demand  (or  trip  tables).    In  the  second  stage,  this  demand  is  loaded  onto  the  transportation  network  in  a  process  than  amounts  to  formal  equilibration  of  route  choice  only,  not of other choice dimensions such as destination, mode, time-of-day, or whether to travel at all (feedback  to  prior  stages  has  often  been  introduced,  but  not  in  a  consistent  and  convergent  manner).  Although this approach has been moderately successful in the aggregate, it has failed to perform in most relevant policy tests, whether on the demand or supply side.

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