NCHRP 17-11 [Completed]
Determination of Safe/Cost Effective Roadside Slopes and Associated Clear Distances
| Project Data
|Roger P. Bligh and Shaw-Pin Miaou
Background: The clear zone concept for roadside design emerged in the mid-1960s, when the idea of a single lateral distance beyond which any potential roadside obstacle did not require removal or protection was introduced. However, acceptance of a single distance for lateral clearance has diminished over time. The Guide for Selecting, Locating and Designing Traffic Barriers (1977) and AASHTO's 1989 Roadside Design Guide provide guidelines for roadside recovery areas based on traffic volume, design speed, side slope, and other roadside conditions. Although these guidelines provide a more realistic approach than the application of a single distance, there are major concerns because the values are based on old studies that used relatively limited data and extrapolated numbers. Experience has also indicated that the recovery area provided along highways is usually not completely clear of all objects and often have side slopes greater than desired. Further, transportation agencies frequently face difficulties in providing desirable recovery areas because of right-of-way constraints or construction costs. Consequently, current practice is to provide an area that provides a reasonable opportunity for a driver to regain a measure of control or to slow an errant vehicle. Updated guidelines are needed to aid designers in determining safe and cost-effective recovery areas, while recognizing the associated constraints.
Objective: The objective of this research is to develop relationships between recovery-area distance and roadway and roadside features, vehicle factors, encroachment parameters, and traffic conditions for the full range of highway functional classes and design speeds.
Tasks: To accomplish the project objective, the following tasks are envisioned: (1) Conduct a critical review of the literature and research in progress on recovery-area distances including the effects of design speed, traffic volumes, side slopes, roadside features, accident numbers and severity, encroachment frequencies and conditions, roadway geometrics, and vehicle factors. (2) Conceptualize a set of relationships for recovery-area distances that relate the variables identified and provide a basis for improved procedures for use by highway designers to determine safe, cost-effective roadside recovery areas. Assess the adequacy and availability of existing data to establish the recovery-area distance relationships conceptualized. (3) Develop relationships for recovery-area distances using the existing data and identify further data collection and analyses needed to strengthen or support the relationships derived from existing data. (4) Prepare a detailed work plan describing the rationale, methods, information sources, data-collection plans, schedule, budget and any special facilities, equipment, or arrangements required to develop and present updated recovery-area distance relationships. (5) Prepare an interim report that summarizes the efforts and findings for review and comment. (6) Collect and analyze data in accordance with the approved work plan. (7) Develop the updated recovery-area distance relationships and conduct a sensitivity analysis of the individual components of the relationships. (8) Conduct an extended review of the relationships by highway designers selected by the project panel. Evaluate the review comments, recommend modifications to the relationships and the report, and make the revisions as directed by the panel. (9) Prepare a final report that describes the entire research effort.
Status: The project has been completed and the research is being continued under NCHRP Project 17-11(02).