The development of a robust, unified systems architecture is an important problem in IVHS technology. This paper presents a sketch of a general architectural framework within which IVHS systems can carry out a wide range of management and control functions. The most important aspect of the work reported here is the definition of two parallel and compatible architectures suitable in the first case for ATMS and ATIS functions, where the driver controls the vehicle, and in the second case for AVCS functions, where the vehicle is under automatic control.The tasks that must be accomplished within either architecture are differentiated across four dimensions: function - the functions range from stabilizing individual vehicles along nominal trajectories to adapting traffic flows to changing demands; time scale - the frequency of decisions and responses varies from under 1 s for continuous control of vehicles to several hours for network flow optimization; spatial scope - the impact of a control action can vary from a single vehicle to the traffic in the entire network; information span - satisfactory accomplishment of the task will require information ranging from that referring to a single vehicle to that which spans system-wide flows. The architecture that we outline incorporates a hierarchy of five layers. This hierarchy helps to formulate a structured, modular approach to the development of IVHS because: The hierarchy satisfactorily resolves all four dimensions of difference in the tasks. Each layer presents a standard reference model to the layer above it. This provides a “clean” interface between layers, and the design of each layer can proceed independently using the reference model of the layer below. When standardized, the reference model serve as IVHS open systems architecture. Communication takes place only between adjacent layers and between peer layers. This will help specify the communication capabilities needed to support the control system. For the ATMS and ATIS functions, in which the driver is in control of the vehicle the tasks are arranged in the following five layers: the physical, regulation, planning, link and network layers. For the most advanced AVCS functions, in which driving tasks are fully automated, the five layers are: physical, vehicle regulation, coordination, link and network. The corresponding layers in the two architectures are functionally similar, although the reference models are significantly different.The functions and the information requirements of the two architectures are sufficiently similar that we strongly urge that future work aimed at successive refinements of either architecture should insist on a graceful transition to the other architecture. In practice, this is most likely to mean that the ATMS and ATIS systems should be designed to accommodate the extensions to the additional features envisaged in a fully automated AVCS system. The modular, hierarchical nature of the architectural framework we have proposed makes it possible to do so from the start, before the AVCS details are fully worked out, and without significant additional cost.The basic motivation for this work is to invite discussion on IVHS architectures from relevant participants including transportation agencies, automobile manufacturers, control and communications equipment developers, and the research community. We have deliberately sketched an idealized portrait of the fully automated AVCS scenario and avoided the important concern5 of system evolution in order to sharpen discussion.