  The advances in information and communication technology are changing theway
people move. Companies that offer demand-responsive transportation serviceshave
the opportunity to reduce their costs and increase their revenues
throughcollaboration, while at the same time reducing the environmental impact
of theiroperations. We consider the case of companies, offering a shared taxi
service, thatare involved in horizontal collaboration and present mixed integer
programmingmodels for the optimization of their routes that embed constraints
aimed at bal-ancing the workload exchange. These constraints bound the
imbalance in terms oftraveled time and/or served customers to be less than
thresholds agreed in advanceby the companies. We also present a heuristic
algorithm and show the benefits ofthe collaboration.
