Description: | 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.
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Related 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.
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