MASH Performance Evaluation of Safety Grates for Culverts

Culverts create numerous roadside hazards along our nation’s highways. Further, these culverts can produce serious accidents when struck by an errant vehicle. Safety treatments for roadside culverts include extending the culvert out of the clear zone, shielding the culvert with guardrail, and making the culvert traversable.

Extending a culvert out of the clear zone often requires large amounts of fill material to reconfigure the roadside slopes adjacent to the roadway. Not only is the extension of the culvert and placement of fill costly, but the reconfigured slopes often produce complicated geometries which can cause errant vehicles to roll over. Using guardrail to shield traffic from culverts also has some disadvantages. Guardrails cannot normally be placed near a culvert because of the steepness of most roadside slopes. As a result, long guardrail installations are usually needed to protect motorists from cross-drainage culverts. This type of installation produces numerous guardrail crashes for every culvert impact that is prevented. Although making a culvert traversable can adversely affect hydraulic efficiency, the cost of this type of treatment is normally modest, and it does not increase the number of crashes by enlarging the hazard.

Not surprisingly, studies of the benefits and costs of safety treatments for cross-drainage structures have indicated that the culvert grates often provide both the least costly and the safest treatment for cross-drainage culverts. However, all of these studies are based upon the basic assumption that grates can make culverts safely traversable when installed on any traversable slope. Although crash testing has been conducted on a number of cross and parallel drainage culverts, only two tests have been conducted to NCHRP 350 on cross-drainage structures and no testing beyond passenger cars at 50 mph has been conducted on parallel drainage structures. Additionally, no testing or modeling has been done on skewed cross-drainage pipe installations. It is necessary that previous research be validated with the latest fleet of vehicles represented in Manual for Assessing Safety Hardware (MASH) in order for the American Association of State Highway Transportation Officials (AASHTO) Technical Committee on Roadside Safety (TCRS) be able to update guidance provided in the Roadside Design Guide and in accordance with the vision statement in their strategic plan, “lead roadside policy development, support safety innovations, and be an information resource to promote a decline in roadway departure related deaths and incapacitating injuries.” Updating this guidance will benefit highway practitioners in providing cost effective safety treatments and reduce the number of roadside hazards.

The objective of this research will be to evaluate slopes, vehicle stability with regard to slopes and grate designs using the latest computer models and followed up with full scale crash testing using MASH vehicles. Parallel drainage structures typically used for approach roads have never been evaluated with a vehicle having a higher center of gravity than a passenger car nor at speeds exceeding 50 mph. Cross drainage structures have not been evaluated with the MASH pickup truck which has a higher center of gravity and greater mass than the older NCHRP 350 pickup. The results of this research will be used to update guidance in the AASHTO Roadside Design Guide, the primary resource used by highway practitioners regarding issues of roadside safety. Completion of this research will provide cost effective solutions for mitigating the hazards created by pipe culvert ends along the roadside, thereby improving safety to the traveling public.

The final deadline for implementing the joint FHWA/AASHTO MASH Implementation plan is December 2019. Although the proposed research will not meet this deadline, it is reasonable to assume that FHWA would grant an extension for the use of older technology for culvert grates until the results of this study are published. Failure to commence this research could require that culvert ends located within the clear zone would now have to be shielded with long, expensive runs of guardrail. These runs of guardrail must be long enough to shield a culvert end from all reasonable trajectories. Since guardrail itself is a hazard, this results in significantly longer areas of exposure to errant vehicles, thereby increasing the number of crashes. Because this research will provide practitioners a much more cost effective solution for mitigating culvert ends along the roadside, states will be very likely to implement this research.

The most comprehensive study of the safety grates was published in by Ross, et al. in 1982 (1). Unfortunately, this research was conducted under safety performance 2 evaluation guidelines contained in Transportation Research Circular 191. Testing for cross-drainage structures was limited to small and heavy passenger cars and it was limited to roadside slopes of 1v:5h. Although computer simulation modeling indicated that the safety grates could be effective on steeper slopes, no crash testing was conducted to verify performance. Crash testing for parallel drainage structures was limited to small and heavy passenger cars at a top speed of 50 mph. It was found that a passenger car could traverse a berm replicating a cross-road with 1v:6h slopes despite becoming airborne.

Another study was undertaken to examine the effectiveness of cross-drainage culvert grates when installed on slopes as steep as 1v:4h, Bryden, J.E. (2). Unfortunately, this study incorporated the use of 1 inch diameter rebar spaced 12 inches apart. Although this culvert grate was found to provide adequate safety performance, the close spacing of the grating makes it much more likely to snag debris and clog the culvert pipe. As a result of the potential for producing localized flooding and allowing water to flow over the road surface, this culvert grating system has not gained wide spread acceptance.

In recognition of the need to examine the safety performance of cross-drainage culvert grates when installed on roadside slopes as steep as 1v:3h, the Midwest States Pooled Fund funded a study to conduct computer simulation, develop a grate design, and conduct two full scale crash tests under NCHRP 350 (the standard at the time the study was conducted), Sicking, et al., 2008 (3). A grate was designed to cover a simulated 20 ft. by 20 ft. opening using 4 inch diameter schedule 40 pipes spaced at 30 inches and placed on a 1v:3h slope. Two full scale crash tests were conducted, a 4500 lb. pickup at 61 mph and an angle of 25.4 degrees and a 1800 lb. car at 61 mph and an angle of 18.7 degrees. Both vehicles successfully traversed the grate meeting all of the salient criteria under NCHRP 350. Unfortunately, no subsequent crash testing has been conducted on cross-drainage structures to date, either to MASH 2009 or MASH 2016. Parallel drainage structures have only been tested with small and large passenger cars and only at 50 mph or less.

Culvert grating guidelines developed by Ross et al., 1982, (1) were included in first edition of the AASHTO Roadside Design Guide (RDG) published in 1996 and have remained essentially unchanged except for a reference to the research conducted by the Midwest States Pooled Fund in 2008.

(1) Ross, H.E., Jr., Sicking, D.L., Hirsch, T.J., Cooner, H.D., Nixon, J.F., Fox, S.V., and Damon, C.P., Safety Treatment of Roadside Drainage Structures, Transportation Research Record No. 868, Transportation Research Board, National Academy of Science, Washington, D.C., 1982.

(2) Bryden, J.E., Evaluation of a Culvert-End Safety Grate, Report No. FHWA/NY/RR-90/151, Engineering Research and Development Bureau, New York State Department of Transportation, August 1990.

(3) Sicking, D. L. , R. W. Bielenberg, J. R. Rohde, J. D. Reid, R.K. Faller, and K. A. Polivka. Safety Grates for Cross-Drainage Culverts. Transportation Research Record 2060-08. TRB, National Research Council, Washington, DC, 2008.

The primary product of this research will be an updated section in the AASHTO Roadside Design Guide, the premier resource for highway practitioners for implementing roadside safety strategies. Until that guidance can be updated in the AASHTO Roadside Design Guide, the results of this research can be published by TRB and presented to various meetings of highway practitioners including the TRB Annual Meeting, the summer meeting of the TRB Committee for Roadside Safety Design (AFB20), Task Force 13, etc. Information will be shared with industry so they can manufacturer culvert grates.