FHWA has sponsored the development of an expert system for diagnostic review of at-grade intersections on rural two-lane highways as a component of the Interactive Highway Safety Design Model (IHSDM). This system, the Intersection Diagnostic Review Module (IDRM), has been developed because conventional design practices and design review procedures often fail to explicitly address the safety consequences of geometric design decisions. In particular, combinations of geometric features may pose safety problems that current design policies do not address (e.g., an intersection in combination with a sharp horizontal curve and a steep grade). The expert system allows such problems to be identified and evaluated in an automated and organized fashion. The structure and knowledge base of this expert system are presented. The system identifies potential safety problems in an intersection design by applying decision rules based on models that quantify the extent to which particular problems may or may not be present. Established geometric design models (such as the sight distance models used in current design policies) as well as new models that have been developed from literature synthesis and judgments by geometric design and safety experts are included. The models that IDRM uses to identify potential safety problems and the threshold values used to determine whether particular problems exist are described. IDRM provides a series of advisory messages indicating design features that should be reviewed for potential safety-related concerns. The advisory messages are based on the nature and extent of concerns identified by comparison with established threshold values.