Park-and-ride programs have been expanding for several
years.  For example, in Virginia they
continue to gain popularity among commuters in both major metropolitan areas
and even in rural areas. 

To what extent have park-and-ride programs influenced
VMT/VHT and emissions as well as travel behavior?  Essentially, what would be the difference in
VMT and emissions if park-and-ride users had simply driven all the way to their
final destinations (e.g. pre-park-and-ride conditions) instead of making part of
the trip by automobile and the final leg of the trip by transit, vanpool, etc.?
Finding results on this could be accomplished by compiling park-and-ride
on-board user survey results and creating two estimated models with one
indicating the actual trips being made (e.g. origin to park-and-ride destination)
and the other indicating a full theoretical auto trip (e.g. completing a full
trip from origin to final destination by auto without the trips being
intercepted by park-and-ride). 
Conceptually, the difference in these two estimates would be a change in
VMT and emissions.