Most Probable Travel Trip Table

The Gambler

You've got to know when to hold 'em
Know when to fold 'em

Travel Demand Models Forecast the “Most Probable” Trip Table,
They Don’t Forecast the Only Possible Trip Table

Because the trip tables that are produced by travel demand models have precise values, this is often confused with being the only answer.  When the US Census says that an average household has 2.58 people this does not mean that this is the only possible answer.  This precise value does not mean that it is the only value that you can find for a household.  In fact, you can be certain that you will find absolutely no households with exactly 2.58 people, unless for some reason you are counting children as fractions of adults☺.  But if you are planning for the infrastructure needed to serve households, this average value is more useful than having no information at all.

For the same reason Las Vegas casinos want to know the average outcome of a game of chance in order to set the odds.  They don’t expect to know the outcome of each single game of chance, they want to know the probability of the outcomes in the long term so they can set the odds. They make money by knowing the average outcomes, including the most probable outcome, not the outcome of each game.

Trip tables that are computed in travel demand models are also the most probable outcomes. Whether it is a table produced by Iterative Proportional Fitting (IPF)/Frataring; Gravity Model Trip Distribution; Logit Mode Choice; etc., what may appear to be a single precise trip table is in reality only the most probable trip table.  And just like the Las Vegas casinos use probability and statistics to come up with the probable outcomes of games, not by testing an infinitely large number of games, the choice models in travel demand models are also derived from probability and statistics. A difference is that every possible outcome in casino game is known.  The odds can be quoted because for example you know that no matter how many times you roll a pair of six sided dice, that there are only 36 possible combinations that will appear.  

When applying statistics in choice models, the most probable outcome is computed, without having to know every possible outcome.   To use the technical terms, the most probable mesostate (i.e. outcome, for example, in “craps” rolling a seven) is selected without having to know how many different microstates (i.e. ways to reach that outcome, for example in a game of “craps”, there are six ways to roll a seven) that there are in that mesostate.  Given that the number of cells in a trip table is seldom known before a transportation study is prepared, and you have to know that number to even compute the possible mesostates (i.e. outcomes),  that is perhaps the best that can be expected. Travel demand forecasts will be the most probable outcome, but they are not, and never could be, a guaranteed outcome.  But Las Vegas seems to do all right for itself by only knowing the most probable outcomes, and it can be expected that those using the trip tables that are output from travel demand models will have the same luck.