Cost-Effectiveness Comparison of Cable Guardrail vs. W-Beam Guardrail
TRP-03-007-82 (Cable Vs. W-Beam)
Edward Post, Patrick Chastain
The Nebraska Department of Roads, NDR, has been involved in a review of its policy on the use of guardrail. The purpose of the NDR review was to develop a revised policy which more directly takes into consideration the effectiveness and benefits as well as the construction and maintenance costs of guardrail improvements and alternatives to guardrail installation such as flattening the embankment front slope. The "effectiveness" of an improvement is defined as the annual reduction in the number of injury accidents, whereas, the "benefit" of an improvement is defined as the annual reduction in the accident societal costs. Because of certain advantages of cable guardrail in comparison to W-beam safety guardrail, the NDR has been investigating the possibilities of using a much larger percentage of the cable guardrail. The research documented in this report was conducted in support of this policy revision effort. The computer program in this HP&R study was developed to expedite the lengthy and tedious cost-effectiveness and benefit-cost computations for (a) installing cable guardrail on roadside embankments, (b) installing W-beam safety guardrail on roadside embankments, or (c) making embankment modifications. The findings in this study showed that the installation of cable guardrail was, in all situations investigated, more attractive than the installation of W-beam safety guardrail. In fact, in some situations, it was found that cable guardrail was attractive, whereas W-beam guardrail was not attractive. The findings in this study also showed that the configuration of the roadside embankment was a very significant factor in justifying the installation of guardrail. It is to be emphasized that the findings in this study were based on the assumption that there were no "point" hazards on or near the embankment such as culverts and trees. Just as the configuration of the embankment was found to be important in determining the need for the installation of guardrail, it is the opinion of the researchers that it is of equal or greater importance that point hazards also be taken into consideration in future developments of the program. The work accomplished in this study has demonstrated that the cost-effectiveness and benefit-cost computer program shows great potential in providing highway engineers and administrators in Nebraska with a managerial tool for evaluating spot safety improvement projects and design projects in order to realize the greatest return on the investment made to reduce injury accidents. Because of the ease and rapidity in using the computer program to conduct a detailed analysis of each roadside embankment situation, it is the opinion of the researchers that there is no real need to attempt to develop generalized guardrail guidelines and policies. It is therefore recommended that NDR incorporate the computer program into their Road Design System currently being used.
Computer Program, Cost-Effectiveness, Cable Guardrail, W-beam Guardrail