We describe Trafficopter, a multi-agent system that collects and propagates traffic information in an urban setting using distributed methods. Agents into the vehicles themselves collect and propagate traffic-related information in a decentralized, self-organizing fashion with no single point of failure. The ideas in this system are the use of the vehicles/agents themselves as a way of collecting traffic data and the way those data are distributed to the interested vehicles. The tools used are a traffic simulator and a set of PDAs/WindowsCE-based terminals equipped with GPS and wireless transceivers. The simulator is used for the investigation of the validity of traffic control and information propagation algorithms in a distributed environment and the WindowsCE terminals for applying the above ideas into the real world.