carpooling makes no difference
What we find is that as you increase a city's stock of light rail or bus cars that there's no impact on the amount of driving. That sounds sort of surprising, but it's logically consistent with the other result that we have about the affect of roads on driving. What we find is that as you add roads to a city, those roads get filled up so there are people waiting to use that capacity.... As you add a little bit more capacity it gets filled up.
http://www.npr.org/2011/07/09/137708751/more-roads-may-pave-way-to-more-traffic
It's not completely laid out in this quote, but the bottom line is that as you add capacity to the road system, either by directly increasing road capacity (more lanes) or by removing people from the road (public transit) the road system "fills up" again by people making more trips, or moving into the areas that have become more reachable, etc.
One corollary of this is that carpooling will make no net difference to road congestion, i.e. will not reduce the total amount of driving.
p.s. The title of the NPR story is "More Roads May Pave The Way To More Traffic" but it's clear when you listen to it that the title should have been "More Roads Will Pave The Way To More Traffic".
