Due to their proven ability to reduce fatalities and injuries as well as their initial low cost, cable barrier systems are rapidly being deployed along the roadside and in the medians of our highways. As the use of these systems has increased, experience has indicated various factors that influence system performance as well as gaps in knowledge. As the number of installations escalates around the country, there is a need to formulate guidelines and standardized practices for cable barrier systems.
A variety of cable barrier systems (proprietary and generic) have been deployed across the country. In general, issues have emerged concerning performance characteristics. In addition, design differences among various available systems contribute to different performance characteristics, implementation, and maintenance requirements. A better understanding of the link between placement and performance of these systems is needed to ensure that they will meet expectations. As a result research is necessary to identify both commonalities and differences, to address knowledge gaps, and to provide guidance that will optimize performance of these systems for different field conditions.
The objective of this research is to develop guidelines for the selection, use, and maintenance of cable barrier systems. The scope of the project includes all cable barrier systems for median and roadside applications. In developing guidelines, the following items shall be addressed:
(a) The range of cross sections (slope, width, shape, etc.) where a cable barrier system is and is not appropriate.
(b) The range where 3- and 4-cable barrier systems are appropriate.
(c) The lateral placement with respect to performance and maintenance.
(d) Construction and maintenance tolerances.
(e) Positioning in proximity to slope changes, drainage structures, raised curbs, fixed objects, and other barrier systems.
(f) Practices regarding interconnection with other barrier systems (particularly existing beam guardrail - including terminals, bull-nose end treatments, bridge approach guardrail, cable anchor locations).
(g) Maintenance guidance to ensure the safey performance of cable barrriers.
(h) Guidance on how to transition from one side of the median to the other.
(i) Placement of cable systems on vertical and horizontal curvatures.
(j) Guidance on types of wire rope.
(k) The differences between high-tension cable barrier systems and low-tension systems and their cost, performance, and maintenance.
(l) Benefits, costs, and construction requirements of different post installations, including direct driven posts, sleeved soil plate designs, and concrete sleeves (cast in place and pre-cast). Examine cost of installation, repair times, performance in different soil or mow strip configurations, climates, and placement on slopes and shoulders.
(m) Design and placement of cable anchors and the influence on cost, performance, and maintenance. Identify tradeoffs with protecting the anchors from impact such as reduced potential for long sections of the system being out of service after impact versus likelihood of more severe injury from striking the shielding used to protect the anchor. Address factors influencing anchor design and spacing and the impact on installation costs, maintenance, and system performance.
(n) The influence of different soils and climate on cable end-anchor design.
(o) The identification of variables (such as spacing between anchors and posts) that affects in-service performance characteristics (such as cable deflection) that differ from crash test results.
Accomplishment of the project objective will require at least the following tasks.
Phase I (1.) Conduct a literature review of recent studies, including international studies, relating to cable barrier systems. (2.) Identify, collect, and review state guidelines and in-service evaluations for cable barrier systems. Additionally, collect and review manufacturer’s design, installation, and maintenance documentation for all proprietary cable barrier systems. (3.) Prepare an interim report that summarizes Tasks 1 and 2, identifies additional information needed to develop the guidelines, provides a revised work plan detailing the scope of Phase II, and includes a detailed outline of the proposed guidelines. (4.) Meet with the NCHRP panel to review the Task 3 interim report approximately 1 month after its submittal. Submit a revised interim report addressing the panel’s review comments.
Phase II (5.) Execute the approved Phase II work plan. Quarterly Progress Reports shall include incremental reports of technical progress. (6.) Prepare draft guidelines, detailing the entire range of typical installations for cable barrier systems. (7.) Formulate and execute a plan to solicit feedback on the draft guidelines, incorporating review by state agencies and industry representatives. Synthesize the comments received in the external review and recommend revisions to the guidelines for the NCHRP’s approval. (8.) Submit a final report documenting the entire research effort including the revised guidelines as an appendix. The final report should document limitations on analysis and sources of the material presented, and cite needs for future research.
Status: The project was completed in November of 2011, and the report was published in August 2012 as NCHRP Report 711, Guidance for the Selection, Use, and Maintenance of Cable Barrier Systems.