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92 CHAPTER 7 SUMMARY AND CONCLUSIONS INTRODUCTION The result of much of this early testing and analysis was a general agreement that curbs in front of the guardrail could The results of the studies identified in the literature and the cause vaulting. If curbs were required for drainage purposes, parametric analyses conducted in this research were synthe- the only alternative was to place the curb behind the face of sized in order to develop a general set of guidelines for the the barrier. This arrangement shields the curb from the impact design and installation of curbs and curbbarrier systems along while allowing the curb to channel runoff. The idea was to roadways with operating speeds greater than 60 km/h. The locate the curb such that minimal interaction between the guidelines are based on the results of both computer simulation vehicle and curb occurred. This worked well with lighter and full-scale crash tests. The study involved the analysis of vehicles such as the 820-kg small car, but did not prevent vehicles traversing several commonly used curb types under a vehicle-curb interaction for the larger cars that have a mass variety of impact conditions, as well as the analysis of vehicle of over 2,000 kg unless the guardrail was retrofitted in some impact into various curbguardrail combinations. The research manner to strengthen it and minimize guardrail deflection. presented herein identified common types of curbs that could To circumvent the problem, one option that was considered be used safely and effectively on high-speed roadways and also was to use a low-profile curb underneath the guardrail to identified the proper combination and placement of curbs and minimize the effects that the curb would have on vehicle barriers that would allow the traffic barriers to be effective, i.e., trajectory if the wheels of the vehicle managed to make contact safely contain and redirect an impacting vehicle. with the curb during impact. Tests were conducted by various organizations in which a low-profile curb was placed behind the face of the guardrail. This design proved successful in SUMMARY OF PREVIOUS tests with the larger cars while tests involving pickup trucks RESEARCH STUDIES resulted in success in some cases and failure in others. In An in-depth review of published literature was conducted to cases where the test was a failure, it was not clear whether the identify information pertinent to the design, safety, and func- failure was induced by vehicle-curb interaction or simply tion of curbs and curbbarrier combinations. The studies found caused by inadequate barrier performance. It was apparent, in the literature used a variety of vehicle types including small however, that curbbarrier systems pose a much greater haz- cars, large cars, and pickup trucks. It was found that both the ard to pickup trucks in high-speed impacts than they do to cars large and small cars crossing curbs less than 150 mm high in and also that much more information regarding pickup impact into curbbarrier systems was needed. a tracking manner are not likely to cause the driver to lose con- trol of the vehicle or cause the vehicle to become unstable unless a secondary impact occurs. The dynamic response of a SUMMARY OF CURRENT RESEARCH pickup truck crossing over curbs, however, had not been eval- uated in previous studies with either full-scale tests or com- FEA was used in this research to conduct a parametric puter simulation and was thus unknown. investigation involving a 2000-kg pickup truck impacting Although errant vehicles leave the roadway in a variety of various curbs and curbbarrier combinations to determine orientations, it is assumed that the majority of these vehicles which types of curbs are safe to use on higher-speed road- encroach onto the roadside in a semicontrolled tracking man- ways and proper placement of a barrier with respect to curb- ner. In such cases, the left or right front bumper would be the ing such that the barrier remains effective in safely contain- first point of contact with a roadside object in an impact event. ing and redirecting the impacting vehicle. The curb types used The position of the bumper upon impact has, therefore, been in the study included the 150-mm AASHTO Types A, B, and a primary concern involving impacts with longitudinal traf- D; the 100-mm AASHTO Types C and G; and the 100-mm fic barriers, where it has been assumed that the position of the New York curb. The roadside safety barrier used in the study bumper during impact is a reasonable indicator of vehicle was the modified G4(1S) guardrail with wood blockouts, one vaulting, or underriding the barrier. of the most widely used guardrails in the United States.