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173 CONCLUSIONS The objective of this project was to develop recommended guidelines for the selection of Test Levels 2 through 5 bridge rails considering in-service performance, site and traffic conditions such as traffic volume, PT and land use under and around the bridge. Currently, only general guidance is available from FHWA and AASHTO on when to use one of the six test levels identified in MASH. State DOT policy makers, bridge engineers and highway designers have little basis for their decisions aside from the general philosophical guidance contained in Chapter 7 of the Roadside Design Guide and Chapter 13 of the AASHTO LRFD Bridge Design Specifications. This project filled that gap using a risk-based approach based on risk and cost- benefit analyses performed with version 3.0.1 of the Roadside Safety Analysis Program (RSAPv3). The results of this project were selection guidelines for choosing the appropriate test level for bridge railings based on site and traffic conditions as well as the observed crash performance of common bridge railings. These selection guidelines, as presented earlier, will be useful to designers and state DOTs for selecting the appropriate bridge railings based on particular site conditions. The resulting selection guidelines are based on the risk of observing a severe or fatal crash during the 30-year life of the bridge railing less than or equal to 0.01 for a 1000-ft long bridge railing. The guidelines explicitly use the traffic volume and percent of trucks as well as the geometric characteristics of the bridge like the horizontal curvature, grade and number of lanes to estimate the likely number of vehicles that will leave the traveled way and strike the bridge rail. The crash performance characteristics of modern crash tested bridge railings were included by examining and modeling real-world crash data to predict the occupant injury, post- impact trajectory and probability of penetrating, rolling over or vaulting over the bridge railing. The area around and surrounding the bridge has been characterized into three categories: low- hazard, medium hazard and high hazard. Low-hazard areas are those where penetrating the bridge railing places only the occupants of the impacting vehicle at risk whereas high-hazard areas have the potential for catastrophic loss of life. The selection guidelines use these characteristics of the bridge, traffic and surrounding area as input and present a recommendation for the appropriate MASH test level bridge railing that will result in a risk equal to or less than 0.01 of observing a severe or fatal crashes over the 30-year life of 1,000-ft of the bridge. These selection guidelines, therefore, make choosing the appropriate bridge railing a function of the characteristics of the bridge and the potential for catastrophic harm if the bridge railing is penetrated. In developing these selection guidelines several areas for improvement were identified that would require further research. First, little is known about the nature and character of vehicle encroachment at high traffic volumes, especially as service level conditions degrade from the free-flow. A better understanding of encroachment rates at poor service level conditions and high traffic volumes would help make these guidelines applicable to a broader range of traffic conditions, especially on heavily traveled urban corridors. Second, there is relatively little known
174 about the encroachment characteristics of heavy vehicles. This project used what little data is available to estimate the encroachment rates and trajectories of heavy vehicles but there is a need for a more comprehensive approach to predicting the frequency and extent of heavy vehicle encroachments. This research was focused exclusively on selecting an appropriate bridge railing but it is also important to ensure that the guardrail-bridge rail transitions, approach guardrail and terminals are also adequately designed. Ultimately, the designer is responsible for the complete safety system on the bridge not just the selection of the bridge railing test level so further research aimed at better specifying how these test level selections should be integrated with the transitions and approach guardrails is a logical next step. While additional research in these areas would help to extend the applicability of these selection guidelines, the guidelines presented herein are a good first step since they should adequately account for the majority of bridge railing selection situations.
