Recommended Guidelines for Curb and Curb-Barrier Installations (2005)

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

93 Each component of the guardrail model was validated both quantitatively and qualitatively with laboratory tests, with the exception of the anchor system for which no test data were available. The modified NCAC C2500R (reduced ele- ment) pickup truck model (i.e., model with modifications made to the suspension system by WPI) was used to simulate the impact of a 2000-kg pickup truck. The NCAC C2500R model had been widely used in previous studies to analyze vehicle impact into roadside barriers and therefore the model had been generally debugged. The accuracy of the model’s results was quantified prior to being used in this study. The model was first used to simu- late a 2000-kg pickup impacting the modified G4(1S) guard- rail at 100 km/h at an angle of 25 degrees. The results were validated by comparing them to a full-scale crash test docu- mented in the literature, and it was concluded that the model provided realistic behavior of both the guardrail and vehicle in such an impact event. The validated model was then used in a parametric analy- sis to investigate the effects of various curb types in tracking impacts with a 2000-kg pickup truck on the stability and tra- jectory of the vehicle during simple curb traversals. The para- metric analysis involved six curb types (AASHTO Types A, B, C, D and G and the 100-mm New York curb), two impact speeds (70 and 100 km/h) and three impact angles (5, 15, and 25 degrees). The model was also used in a parametric study to investi- gate the crashworthiness of curb–barrier combinations in tracking impacts with the 2000-kg pickup truck. The para- metric analysis involved the modified NCAC C2500R pickup truck model impacting the modified G4(1S) guardrail model (1) at impact speeds of 70, 85, and 100 km/h; (2) at an impact angle of 25 degrees; (3) and at offset distances from curb to barrier of 0, 2.5, and 4 m. The results of the curb traversal study indicated that the stability of the pickup truck was not compromised in tracking impacts, but the trajectory of the front bumper was sufficient to imply a risk of barrier over- ride when a standard strong-post guardrail is placed anywhere from 0.5 m to 7.0 m behind 150-mm high curbs or 0.6 m to 7.0 m behind 100-mm high curbs. The results of the pickup truck model impacting various curb–guardrail combinations confirmed that the presence of curbs was potentially hazardous. The results of the paramet- ric study were used to identify certain combinations that were more likely to result in acceptable barrier performance and those more likely to result in unacceptable barrier perfor- mance, and guidelines defining proper curb type and barrier placement were presented. It should be noted that even cases identified as being successful resulted in poorer performance of the guardrail and a higher risk of injury for the occupants of the vehicle than was the case when the curb was not pres- ent. These guidelines were validated by full-scale crash tests of curb–guardrail combinations. CONCLUSIONS The result of the foregoing analyses and testing was the development of recommended guidelines for the use of curbs. The results of the study of tracking vehicles traversing curbs where guardrails are not present indicated that the front bumper trajectory is only slightly affected by the impact speed, impact angle, and slope of the curb face. The most sig- nificant factor influencing the trajectory and vehicle stability in these tracking impacts is the height of the curb. Vehicles also often interact with curbs in a nontracking con- figuration. A tripping risk index (TRI) was developed to quan- tify the performance of curbs in nontracking situations. The index was developed using full-scale live-driver curb traversal tests and finite-element simulations of a 2000-kg pickup truck traversing a variety of curbs in nontracking impacts. TRI val- ues above 45 were considered to indicate that vehicles were at high risk of tripping whereas TRI values less than 20 pre- sented a very low risk. TRI values between 20 and 45 were considered moderate. The best curb evaluated in this study was the New York curb which resulted in a TRI of just over 12. The AASHTO Types C and G curbs presented moderate risk on high-speed roads and the AASHTO Types B and D presented high risk for high-speed roads. When curbs must be used on high-speed roads, the shortest possible curb height and flattest slope should be used to minimize the risk of trip- ping the vehicle in a nontracking collision. Guidelines for use of curbs in conjunction with guardrails were also developed. Any combination of a sloping-faced curb that is 150-mm or shorter and a strong-post guardrail can be used where the curb is flush with the face of the guardrail up to an operating speed of 85 km/h. Guardrails installed behind curbs should not be located closer than 2.5 m for any operating speed in excess of 60 km/h. The vehicle bumper may rise above the critical height of the guardrail for many road departure angles and speeds in this region, making vaulting the barrier likely. A lateral distance of at least 2.5 m is needed to allow the vehicle suspension to return to its predeparture state. Once the suspension and bumper have returned to their normal position, impacts with the barrier should proceed successfully. For roadways with operating speeds of 70 km/h or less, guardrails may be used with 150-mm high or shorter sloping- face curbs as long as the face of the guardrail is located at least 2.5 m behind the curb. Vehicles traveling at speeds greater than 70 km/h may vault over the guardrail for some departure angles. In cases where guardrails are installed behind curbs on roads with operating speeds between 71 and 85 km/h, a lateral distance of at least 4 m is needed to allow the vehicle suspen- sion to return to its predeparture position. Once the suspension and bumper have returned to their normal position, impacts with the barrier should proceed successfully. Guardrails may

94 be used with 100-mm high or shorter sloping-face curbs as long as the face of the guardrail is located at least 4 m behind the curb. Vehicles traveling at speeds greater than 85 km/h may vault over the guardrail for some departure angles. Above operating speeds of 85 km/h, guardrails should only be used with 100-mm high or shorter sloping-faced curbs, and they should be placed with the curb flush with the face of the guardrail. Above operating speeds of 90 km/h, the sloping face of the curb must be 13 or flatter and must be 100-mm high or shorter. These recommended guidelines should help practitioners select appropriate curb and guardrail combinations at sites where both curbs and guardrails are necessary. Curbs should only be used on higher speed roadways when concerns about drainage make them essential to the proper maintenance of the highway.