MIDWEST STATES POOLED FUND PROGRAM
Safety Investigation & Design Guidance for Curb & Gutter Near Energy-Absorbing Terminals
Sponsoring Agency Code
TPF-5(193) Supplement 62
AASHTO highway design policies discourage the use of curbs along high-speed roadways. This guidance is largely based on the fact that curbs may cause impacting vehicles to become airborne, thus resulting in loss of control by the driver. In the case of a laterally skidding vehicle, a rollover may also be induced upon striking the curb (i.e., tripping). However, safety appurtenances, such as guardrail end terminals and crash cushions, are often placed in combination with curbs. Nonetheless, curbs are often installed along high-speed roadways for several reasons, including restricted right-of-way, drainage considerations, access control and other curb function requirements. In these situations, eliminating existing curbs or laterally offsetting curbs away from the traveled way may represent an expensive or unattainable alternative.
Historically, the safety performance of energy-absorbing guardrail end terminals has been based on the results of full-scale crash tests performed on level terrain. However, very limited research has been performed to investigate the safety performance of these features when installed in combination with curbs. Thus, there is a need to investigate whether curb placement in advance of guardrail end terminals significantly degrades barrier performance as a result of the changes in vehicle trajectory prior to impact. In addition, design recommendations are necessary for determining the safe placement of curb and gutter installed adjacent to energy-absorbing guardrail end terminals.
At this time, no research has been performed on curbs used in conjunction with guardrail end terminals. Nonetheless, previously performed studies related to the interaction between curbs and crash cushions or barriers may provide useful information about the influence that a change in the vehicle trajectory may have on the safety of roadside hardware.
Several years ago and in 1979, CALTRANS researchers investigated the safety performance of sand barrel crash cushions in conjunction with 6-in. high curbed gores. In this study, eight live-driver, crash tests were conducted with small car and large passenger vehicles. These crash tests were performed head-on into curbed gore areas at speeds of 40 and 60 mph. These tests indicated that the highest rise in vehicle trajectory occurred with the small vehicle traveling at 40 mph. This peak rise was 9.5 in. above the top of the gore at a distance of 14.5 ft beyond the nose of the curbed gore. The performance of a sand barrel crash cushion, placed 5 ft back from the nose of the curbed gore, was not appreciably affected. This result was observed when evaluated by a vehicular impact which was deemed to produce the greatest potential for vehicle vaulting (i.e., small car at 40 mph and head-on). For both parts of this study, the raised asphalt concrete gore surface was bounded by a 6-in. high, sloping-face concrete curb, forming a gore about 50 ft long and having a nose radius of 5 ft.
Research has also been conducted to investigate the performance of guardrails placed in front of curbs. Barrier offset away from the curb has been shown to effect system performance through computer modeling and crash testing. Previous work with steel-post, nested W-beam guardrail has shown that a 4-in. high sloped curb with the toe of the curb placed at the front face of the W-beam guardrail is capable of meeting NCHRP Report No. 350 safety requirements. Further research with standard wood-post, W-beam guardrail has shown that a 4-in. high sloped curb with its toe set out 1 in. from the front face of the guardrail is also capable of meeting TL-3 requirements.
Investigation of curb-barrier combinations was also investigated in NCHRP Report No. 537, Recommended Guidelines for Curbs and Curb-Barrier Combinations. This study developed guidelines for the use of curbs and curb-barrier combinations on roadways with operating speeds greater than 37.3 mph. The study recommended that guardrail be installed flush with the face of the sloped curb or offset more than 8.2 ft behind the curb for operating speeds in excess of 37.3 mph. In addition, the study recommended that guardrail should not be offset behind sloped curbs for speeds greater than 62.1 mph.
The recent development and testing of the Midwest Guardrail System (MGS) has demonstrated that this system can be used with a 6-in. (152-mm) tall, American Association of State Highway Transportation Officials (AASHTO) Type B curb positioned 6 in. forward from the front of the face of the guardrail element. Additional research was conducted with respect to TL-3 impacts on the MGS system with larger curb offsets. This research did not yield clear guidance on larger curb offsets for the MGS under TL-3 impact conditions. Thus, the current guidance regarding curb placement near the MGS on high-speed facilities remains the 6-in. offset noted above. This limitation on curb placement is critical as installation of a tangent end terminal within 6 in. of a curb would likely result in the impact head hanging into the roadway. Thus, placement of tangent end terminals adjacent to curbs may require some flaring of the terminal (flares of 1 ft over 50 ft are relatively common in Texas and other states).
The previous research studies provide useful information for vehicular impacts into curbs and its effect on post-impact crash behaviors, vehicle motions, and subsequent trajectories onto nearby terrain where roadside guardrails may or may not be installed. This research approach may be useful for determining whether curbs may also affect the safety performance of crash cushions and guardrail end terminals.
The objective of this research effort is to develop guidance for the safe placement of curbs adjacent to energy-absorbing guardrail end terminals. A combination of computer simulation and full-scale crash tests will be used to identify potential safety hazards, define critical curb and terminal impact scenarios, and select optimal curb placement. The effort will focus on a single, representative energy-absorbing, guardrail end terminal configuration that is selected during the study effort. In addition, the impact conditions for the simulation and crash testing programs will correspond with those published for Test Level 3 (TL-3) in the MASH impact safety standards.
The research project is separated into two phases. As part of Phase I, the research plan will include a literature search of previous barrier and curb studies as well as a review of current energy-absorbing guardrail end terminals. The literature search will focus on guardrail-to-curb combinations, especially acceptable applications of both 4- and 6-in. curbs with W-beam guardrail systems. The effort will identify critical curb parameters when used in conjunction with highway barriers, such as curb height and shape, curb offset, and vehicle type and model. Next, MwRSF will conduct a survey of the Pooled Fund states to determine current practice, guidelines, and standards pertaining to the use of curb with other safety appurtenances found on high-speed roadways. The survey should also identify problems experienced by these agencies and any proposed solutions, such as the flaring of tangent terminals noted above. In addition, the literature search and state survey will provide the basic configuration and parameters for a computer simulation study of curb and energy-absorbing terminal installations.
As part of Phase I and following the literature search and survey of state practices, MwRSF will conduct a series of LS-DYNA computer simulations to determine the performance of energy-absorbing terminals adjacent to curbs. The simulation effort will focus on head-on and oblique impacts near the end of the terminals and be used to determine potential hazards and failure mechanisms, identify critical impact conditions, and suggest potential safety improvements. Parameters for investigation may include curb position, height, and shape, vehicle type (1100C or 2270P), impact angle, impact location on the terminal, and flaring of the terminal. The simulation data would then be used to determine critical impact tests for both 1100C and 2270P vehicles. A summary report would be prepared at the conclusion of Phase I.
If subsequent funding becomes available and Wisconsin DOT personnel deem it viable, then the project could be continued with a follow-on Phase II effort. For this project, two full-scale vehicle crash tests would be conducted using the critical parameters and impact conditions selected through the computer simulation analysis. One 1100C and one 2270P test would be conducted to demonstrate the validity of the computer simulations as well as to provide guidance for further curb and energy absorbing terminal recommendations. The selection of the guardrail end terminal will be made during the Phase I study.
Following the Phase II research effort, MwRSF will submit a final report which documents the research effort. In addition, recommendations will be provided with respect to safe placement of curbs adjacent to energy-absorbing guardrail end terminals.
The development of design guidelines for the safe placement of energy-absorbing guardrail end terminals behind curbs will provide beneficial information for highway designers and engineers and reduce the risk of highway agencies adopting inadequate and potentially unsafe curb-barrier combinations. These guidelines would also serve to reduce inconsistencies in the recommendations from one highway agency to the next, inconsistencies which could be the source of significant tort risk. These guidelines could potentially reduce highway agency expenses associated with curb removal in front of guardrail end terminals if certain combinations are found to be safe and no longer prohibited. In addition to being costly, curb removal is hazardous to both workers who are exposed to highway traffic in construction zones and the motorists who must traverse a restricted travel way. Any funds which can be saved by avoiding curb removal could be used for implementing other cost-beneficial safety improvements.
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