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We have a question regarding the performance of end terminals in conjunction with raised
curbs.  

Currently, we understand that a
typical run of guardrail set at 6” behind the face of curb is acceptable.
However, this curb configuration causes several issues near an end terminal and
conflicts with the Roadside Design Guide’s recommendations for flat grading
surrounding the end terminal. Unfortunately, dropping the curb at the end
terminal location is not always feasible when drainage issues are considered,
so we’re actively looking for solutions where curbed sections are needed (a
frequent scenario).

Do you know of any recent
studies or criteria that address this topic of end terminals working in
conjunction with raised curbs?

We really appreciate your help!

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).

We currently have a project at MwRSF to begin the study of curbs and energy-absorbing terminals that was sponsored by the Wisconsin DOT. The objective of this research effort is to develop guidance for the safe placement of curbs adjacent to energy-absorbing guardrail end terminals. Initially, computer simulation will be used to identify potential safety hazards, define critical curb and terminal impact scenarios, and select optimal curb placement. 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.

Thus, there is currently very little hard guidance one can give regarding curbs and end terminals. In general, shorter, wedge shaped curbs will likely be more forgiving based on previous research with curbs and other barrier types.

Let me know if you need any further information.

We greatly appreciate your response and input. I think that we should be able to work with the past practices you’ve mentioned with possible limitations added.

In the future, we recommend further considering topics of:

1.       Soil heights being raised at the terminal assembly posts as a result of the curb height

2.       Curb effects on vehicle vaulting and stability for end terminal nose impacts at a shallow angle (head-on, moving near-parallel to curb)

Again, thank you for the background and input! As always, we look forward to learning the results of your next study.

Thank you,