Jeff Cardinal in his response to the 'Cracking the Carpool Conumdrum', an excellent story about how SAP used some software from Ridespring to get employees to sign up for ridesharing, said: Half the time I will be running errands at lunchtime or on my way home from work. This makes it difficult to participate in car pool programs. If someone could figure some way to accommodate that situation, participation would increase.
Now a new approach to carpooling called flexible carpooling provides that potential. See
http://www.flexiblecarpooling.org.