Methods, systems, and apparatus, including computer programs encoded on computer storage media, for determining travel information. One of the methods includes obtaining flight information for each of a plurality of users, the flight having a flight identifier and associated with a particular scheduled departure time and departure location; based on the scheduled departure time, obtaining location information for user devices associated with each user of the plurality of users; determining that the respective user devices associated with a first group of users of the plurality of users, has a location associated with the departure location; determining that the respective mobile devices associated with users of the first group are no longer in communication a mobile network; and using the respective times at which the user devices are determined to no longer be in communication with the mobile network along with the scheduled departure time to determine a departure time.