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    I know it exists because I used it once with a student -- which doesn't mean  
I still have it (or that I remember where I got it it).  At the time there was  
nothing remarkably difficult about getting it.

It was point to point for a year with passenger as the unit of analysis.

One big validity problem:  In the real world passenger  passenger traffic is not  
a monotone function of either distance or real traffic:  There isn't much air  
traffic (some but not much) between extremely nearby destinations:  take a cab.  


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