This paper describes the design and preliminary evaluation of the criteria for alerting drivers to a specific set of intersection hazards. The research is being conducted as part of the development of an intersection decision support (IDS) system that uses the sensing and computational technologies of infrastructure-based intelligent transportation systems. The IDS system under consideration is intended to help drivers avoid conflicts with oncoming traffic when they are making left turns under a permissive (i.e., unprotected) green signal. These conflicts account for a significant proportion of intersection crashes and are difficult to mitigate without imposing serious costs and burdens on intersection capacity associated with providing a protected left-turn signal cycle. The human factors and sensing issues that need to be considered in designing the system are discussed and are followed by a description of the logic used to define the gaps in opposing traffic that should be considered adequate for left-turn maneuvers. The simulation model used to evaluate alternative system designs is described, and sample results are shown for evaluation of the effectiveness of a warning under a relatively stressful scenario. The influence of alternative sensor configurations on the effectiveness of the warning is illustrated and indicates the importance of providing information about both the presence and speed of approaching vehicles sufficiently far from the intersection.