It is not easy, however, for an institution of our size, to secure such donations, although we did not try. We did, however, specifically decide to keep all of our motivating schemes positive, so there were no DIS-incentives such as parking fees to discourage driving. That might also become an option in future iterations of the program.
In examining the data we collected, it has become clear to me that the biggest gains in CO2 reductions will be realized by focusing on those making the longest commutes. The vast majority of people who really engaged in the project were the ones who live relatively close to campus to begin with, making biking and walking a real alternative for them, but which saves very little in CO2 emissions because of their proximity to campus. We have many students, and even some employees, who commute 1-3 hours to get to campus (which is possible because of our innovative delivery model whereby most students and many faculty only need to come to campus at most 1-2 days each week). We could have done a lot more outreach and match-making among those who are commuting from the furthest distances.
Another big challenge to the project was the transition from the FirstClass email system to Gmail. In FirstClass, I had the ability (and privileges) to post the ACT Update to a particular conference called AUNE Announcements. This was ideal for ACT communications because upon opening up FirstClass, the ACT Update would appear in a pop-up window, thereby greatly increasing the odds that participants would read about the project and submit their commuter logs. Gmail did not offer any such functionality, and this does explain why the number of logs submitted for the month of April was so small – just two! What we perhaps should have taken the time to do (except that we were all so busy at the time) was to create a Gmail mailing list of all ACT registrants so that we could have individually emailed all of them the ACT Update and commuter log reminders. However, we were all floundering as we tried to learn the new system.
In addition, an oversight in our data collection instrument needs to be pointed out. Although we asked each registrant to select a primary affiliation (student, full-time faculty, part-time faculty, full-time administrator/staff, and part-time administrator/staff), we did not have the students affiliate with an academic department. This would have been useful in terms of determining whether targeted outreach strategies were needed for specific academic departments. We look forward to correcting for this in future iterations of the project. 
On the more positive side, I can say with confidence that carpooling really does build social capital. If you can just get people over that initial resistance to carpooling, they inevitably find that the conversations and social connections with ride-mates really do outweigh the costs of losing a little flexibility and independence. 