NCHRP 17-14 [Final]
Improved Guidelines for Median Safety
| Project Data
||Warren E. Hughes|
Background: Foreslopes of existing depressed medians are often flattened as a safety improvement, assuming that flatter slopes will allow errant vehicles to recover. This assumption is well accepted and supported for foreslopes on the outside of a roadway. For narrow medians without barriers, however, these flatter slopes may increase the potential for vehicles to cross the median. The American Association of State Highway and Transportation Officials' (AASHTO) Roadside Design Guide and Policy on Geometric Design of Highways and Streets contain information that is used in the design and redesign of medians. Much of these information is based on old data and may not reflect today's conditions.
Objective: The objective of this research is to develop improved guidelines for the use of median barriers and for the selection of median widths and slopes on new and existing high-speed divided highways. The guidelines should be suitable for inclusion in AASHTO's Roadside Design Guide and Policy on Geometric Design of Highways and Streets.
Tasks: Research includes the following tasks: (1) Review recent pertinent literature and research in progress on median-related accidents to identify critical parameters and their impact on these types of accidents. (2) Survey appropriate state transportation officials to identify unpublished studies, characterize available accident and highway inventory data (particularly before-and-after data for barrier installations and slope modifications), and ascertain the data's suitability for this study. Identify states that are willing to participate in the data-collection tasks. (3) Formulate data-collection, validation, and analysis plans to determine the effects of barriers and median slopes and widths on accident frequency and severity for all accident types. The data-collection plan should include roadways with a variety of median designs and operational characteristics. Using data that is readily available from three or four states, evaluate the data-collection, validation, and analysis plans. (4) Prepare an interim report summarizing the efforts and findings of Tasks 1 through 3. The report must include a revised work plan and budget for the remaining tasks. The interim report must present a realistic discussion on the potential for achieving the project's objective within the project's time and financial constraints. (5) Execute the approved data-collection and validation plans. (6) Analyze the data from Task 5 in accordance with the approved data analysis plan. Critically evaluate current guidelines. Develop improved guidelines that address deficiencies identified. The improved guidelines should be easily interpreted and expressed in charts and tables. Recommend modifications (charts, tables, and supporting text) to AASHTO's Roadside Design Guide and Policy on Geometric Design of Highways and Streets. (7) Submit a final report that documents the entire research effort and includes the recommended modifications to AASHTO documents as a stand-alone appendix.
Status: The project was delayed for 27 months due to data collection difficulties. The analysis results turned out to be inconclusive and the project oversight panel did not recommend publication. Findings have been provided to subsequent research efforts.