ROADSIDE DESIGN GUIDE
|Publication Date:||1 January 2011|
This Roadside Design Guide was developed by the American Association of State Highway and Transportation Officials (AASHTO) Subcommittee on Design through the Technical Committee on Roadside Safety (TCRS) under the chairmanship of Keith Cota, P.E. This book presents a synthesis of current information and operating practices related to roadside safety and is written in dual units- metric and U.S. Customary. This edition supersedes the 2006 AASHTO publication, which included the update of the Median chapter.
The roadside is defined as that area beyond the traveled way (i.e., driving lanes) and the shoulder (if any) of the roadway itself. Consequently, roadside delineation, shoulder surface treatments, and similar on-roadway safety features are not extensively discussed. Although safety can best be served by keeping motorists on the road, the focus of this guide is on safety treatments that minimize the likelihood of serious injuries when a driver does run off the road.
A second noteworthy point is that this book is a guide. It is not a standard, nor is it a design policy. It is intended to be used as a resource document from which individual highway agencies can develop standards and policies. Although much of the material in the guide can be considered universal in its application, several recommendations are subjective in nature and may need modification to fit local conditions. However, it is important that significant deviations from the guide be based on operational experience and objective analysis.
To be consistent with AASHTO's A Policy on Geometric Design of Highways and Streets, design speed has been selected as the basic speed parameter to be used in this guide. However, because the design speed often is selected based on the most restrictive physical features found on a specific project, reasonable and prudent drivers may exceed that speed for a significant percentage of a project length. There will be other instances in which roadway conditions will prevent most motorists from driving as fast as the design speed. Because roadside safety design is intended to minimize the consequences of a motorist leaving the roadway inadvertently, the designer should consider the speed at which encroachments are most likely to occur when selecting an appropriate roadside design standard or feature
The 2011 edition of the AASHTO Roadside Design Guide has been updated to include hardware that has met the evaluation criteria contained in the National Cooperative Highway Research Program (NCHRP) Report 350: Recommended Procedures for the Safety Performance Evaluation of Highway Features and begins to detail the most current evaluation criteria contained under the Manual for Assessing Safety Hardware, 2009 (MASH). For the most part, roadside hardware tested and accepted under older guidelines that are no longer applicable has been included in this edition.
The TCRS is currently working through a National Cooperative Highway Research Program (NCHRP) research project to update the Roadside Safety Analysis Program (RSAP) with the development of a "window-friendly" version. The RSAP update will be "beta" tested in 2011 and is expected to be available through AASHTO in early 2012, and will be available through a link on the web-based format of this publication.
As mentioned, design values are presented in this document in both metric and U.S. Customary units. The relationship between these values is neither an exact (i.e., soft) conversion nor a completely rationalized (i.e., hard) conversion. The metric values are those that would have been used had the guide been presented exclusively in metric units, while the U.S. Customary values are those that would have been used if the guide had been presented exclusively in U.S. Customary units. Therefore, the user is advised to work entirely in one system and not to attempt to convert directly between the two.
The reader is cautioned that roadside safety policy, criteria, and technology is a rapidly changing field of study. Changes in the roadside safety field are certain to occur after this document is published. Efforts should be made to incorporate the appropriate current design elements into the project development. Comments from users of this guide about suggested changes or modifications that result from further developmental work or hands-on experience will be appreciated. All such comments should be addressed to the American Association of State Highway and Transportation Officials, Engineering Program, 444 North Capitol Street NW, Suite 249, Washington, DC 20001.