Background: Consistency with drivers' expectations is a desirable property of geometric designs. On non-urban, two-lane roads, drivers typically expect to be able to operate their vehicles safely with relatively little mental effort. Geometric features that are atypical, have extreme dimensions, or are combined with other features in unusual ways violate these expectations; such features are termed geometric inconsistencies. Geometric inconsistencies can surprise the driver and reduce the safety of the road. Previous research has identified geometric features that may violate driver expectancy, particularly when they are located close together, such as: (1) horizontal and vertical alignment changes, (2) intersections and driveways both channelized and unchannelized, (3) lane drops, (4) divided highway transitions, (5) lane width reductions, and (6) shoulder width reductions and changes in composition.

While geometric consistency is an issue on all types of roads, this project will deal only with non-urban, higher-speed, two-lane roads. It is expected that future efforts will look at multilane roads, and it is hoped that some of the results of this project will be transferable to those types of roads.

Objective: The objective of this project was to develop guidelines that designers can use to improve the geometric design consistency of roadway features on higher-speed, non-urban, two-lane roads. The guidelines should be suitable for identifying specific problem locations and for analyzing alternative designs for new locations and reconstruction. At the interim meeting of the research team and oversight panel, it was decided that these guidelines should consist of rules for an expert system that could be incorporated into the Interactive Highway Safety Design Model (IHSDM). This approach was taken because it did not appear that quantitative analysis would be productive.

Product Availability: NCHRP Report 502, Geometric Design Consistency on High-Speed Rural Two-Lane Roadways, has been published.

Tasks: To accomplish this objective, the following tasks are envisioned: (1) Analyze, describe, and critique pertinent domestic and international research findings on the basis of applicability, conclusiveness of findings, and usefulness in developing geometric design consistency guidelines and relationships between geometric features, consistency, and safety. Establish working definitions for geometric design consistency and inconsistency. (2) Develop a comprehensive list of geometric design features for non-urban, higher-speed, two-lane roads that can reduce geometric consistency or violate driver expectancy. Identify the most critical roadway features or combinations of features and how they might impact driver performance. Identify the specific design and operational parameters of those features or combinations of features that potentially contribute to inconsistency. (3) Develop measures that can be used to rate or describe the consistency or inconsistency of the features of a two-lane road. Example measures would be traffic conflicts and changes in operating speed. Postulate relationships among the parameters of the features identified in Task 2, these measures, and safety. (4) Formulate a data-collection and analysis plan to develop the relationships postulated in Task 3. The plan should address those critical features or combinations of features identified in Task 2 needing further study. The approach should include crash data analysis and may also include other methods (e.g., simulation, laboratory data, field data) (5) Prepare an interim report summarizing the efforts and findings of Tasks 1 through 4. The report must include a revised work plan and budget for the remaining tasks and an outline for the Task 7 guidelines. (6) Identify the appropriate form and structure for an expert system to evaluate design consistency. (7) Produce the material required to support the expert system. The material will list and provide examples of design consistency problems, critical causal factors, and recommended countermeasures. (8) Test the material developed in Task 7 through an expert review by members of the research team. (9) Further examine the Task 7 material using case studies of existing and proposed roadways. (10) Submit a final report that documents the entire research effort and includes written guidelines regarding the rules from the expert system and functional specifications for its development. The final report should include an updated implementation plan.