In transportation engineering, mode choice refers to a person's decision to take a particular mode of transportation (such as driving, walking, biking, mass transit, private helicopter, etc.) to get where he or she is going.  The resulting percentages of who-takes-what; the pie chart, if you will, is known as the mode split.  The percentage of all travelers taking each mode is known as the mode share for that mode.  (In transportation engineering, as in all engineering ever, we are very precise in our terminology).  For most cities in the United States, it is reasonable to assume a transit mode share of 5% or less, and acceptably conservative to assume none at all.

Of the many different possible mode splits for a given city, two are of particular interest to transportation nerds.  Under Individual Equilibrium (IE), everyone takes whichever mode of transportation happens to be most convenient for him or her at the time.  What this generally shakes out to is that most people drive; only enough people take transit, walk, bike, etc. as absolutely necessary to keep the freeway moving.  It also means that if you were to suddenly cancel all the transit service in a city at once, the freeway and many local streets would completely shut down from the additional traffic until people figured out alternate routes or stopped making as many car trips.  An interesting, if unpleasant, thought.

Under Social Optimum (SO), the mode split is such that all modes operate as efficiently as possible.  Enough people are taking transit that the freeway is actually nice, but the transit system isn't overloaded to the point of bad BO in your face, either.  This does mean that each person may not get where he or she is going quite as fast as preferred, but the travel time averaged over everyone is the lowest.

Do you see the problem here?  In order to improve your transportation system from IE to SO, you have to convince people to do things they don't normally want to do. Like take transit.

Enter the incentives.  Free/cheap bus passes for students/seniors/babies/employees of your company.  FrontRunner group passes (still an unconscionably good deal).  Free bus to BYU football games if you park and eat at University Mall before the game.  We've got to do something to get some of you off the road so that the rest of you can get down the road faster.  That is, until enough of you realize the merits of transit and start taking it on your own.

But who am I kidding?  That won't happen for at least a thousand years.


  1. Nice post. Where do you get the IE/SE idea from? A specific book/paper or your own idea?

  2. This theory is from one of my textbooks (guess I should cite it . . .) but I can't remember the title--I'll get back to you when I find it.