Distances between locations traveled by a carpool driver in a carpooling system may be initially estimated by calculating direct, straight line distances between each of the location points. Travel speeds may also be initially estimated using an expected maximum vehicle speed, which may a maximum speed limit. An estimated travel time may then be calculated from this data to initially designate passengers as eligible or ineligible for carpooling with a carpool driver. Cancellation requests received from participants in assigned carpools may be processed to determine whether the carpool may still be conducted. If so, the carpool may continue. If not, the system may search for alternative assignments before sending any cancellation notifications to the participants. Cancellations may be sent only if the system is not able provide a suitable alternate assignment.