  A Vehicular Ad-hoc NETwork (VANET) is a special form of Mobile Ad-hoc Network
designed to provide communications among nearby vehicles and between vehicles
and nearby fixed roadside equipment. Its main goal is to improve safety and
comfort for passengers, but it can also be used for commercial applications. In
this latter case, it will be necessary to motivate drivers to cooperate and
contribute to packet forwarding in Vehicle-to-Vehicle and Vehicle-to-Roadside
communications. This paper examines the problem, analyzes the drawbacks of
known schemes and proposes a new secure incentive scheme to stimulate
cooperation in VANETs, taking into account factors such as time and distance.
