Present and future automated Intelligent Vehicle Mighway Systems (IVHS) require cooperation and communication between vehicles on the road. An IVHS system can be divided into four layers of control. The regulation layer executes control algorithms that actuate throttle, braking, and/or steering. Control algorithms designed at PATH (Partners for Advanced Transit and Highways) require a 5Oms sensor sampling rate. The regulation layer receives commands from the platoon layer which plans maneuvers such as merging vehicles into a platoon, splitting a platoon of vehicles, and lane changes. An on-board computer executes both the regulation and platoon layer on each vehicle. The link layer may assign a platoon speed and size. This layer and a higher layer, the network layer, are implemented in roadside computers. The focus of this paper is on the regulation and platoon layers. A protocol is proposed to allow the exchange of messages by vehicles performing co-operative maneuvers. The protocol assumes that all of the interacting vehicles have access to a single radio channel. Data is transmitted in frames that are divided into two channels: A synchronous channel used for round robin transmissions or receptions of data needed by the vehicle's regulation layer, and a contention channel used to exchange messages of the platoon layer. The synchronous channel is used to transmit or receive speed and acceleration of each vehicle between its neighbors. The test and implementation of the protocol on an automatically controlled platoon to maintain longitudinal control is described. Recent experiments in two-car platoon systems in PATH demonstrated a successful scheme for inter-vehicle communication . This paper presents another design of a communication task for multi-car platoons and a C-language implementation.