The AASHTO Roadside Design Guide recommends that guardrail be installed with the back
edges of the guardrail post being 2 ft from a slope break. In many mountainous areas or in
locations with tight environmental controls this width is difficult to provide. As a result,
designers often have to make a trade-off between reduced shoulder width and a less than optimal
guardrail placement. The WsDOT Design Manual provides for the placement of the guardrail
post closer to or on slopes as steep as 1H:1V as illustrated in Figure 1.
In an earlier project, TTI researchers conducted two full scale crash tests of a 31-in high
guardrail system placed on 2H:1V slope. The posts were placed 1-ft from the slope break such
that the face of the guardrail was aligned with the slope break as shown in Figure 2. The overall
system drawing is shown in Figure 3.
The crash tests conducted were MASH Test designation 3-11 and MASH test designation 3-10,
which involves a 2270 kg pickup truck and a 1100 kg small car respectively. Both test vehicles
were setup so they impact the CIP of the length of need section at a nominal speed of 100 km/h
(62 mi/h) and a nominal angle of 25 degrees. Each test resulted in vehicle redirected successfully
as shown in the sequential images in Figure 4. The impact severity metrics were within the
acceptable criteria of MASH guidelines for each test. Therefore, these tests passed the MASH
test evaluation criteria and subsequently an eligibility letter is in the process of being issued by
TTI researchers investigated the feasibility of guardrail placement on steeper slopes such as
1H:1V through nonlinear finite element analyses in a subsequent project. Simulations for both
MASH TL-3-10 and MASH TL-3-11 indicated a likelihood of successful outcome of testing for
a regularly spaced posts, 31-in tall W-beam guardrail system. The posts were placed 1-ft from
the slope break such
A design was tested in pool fund project, but the w-beam rail ruptured once impacted by the
small car as shown in Figure 5.
Subsequently, a thrie-beam based design was simulated to increase the strength of the rail
elements. This thrie-beam bases designed showed an increased likelihood of passing MASH tests
3-11 and 3-10. Both the truck (MASH 3-11) and the car (MASH 3-10) showed a reasonable
chance of passing MASH criteria with this new design.
Adding a plate rubrail to the system help with reducing the potential of wheel snagging on the
steel posts as shown in Figure 6 and Figure 7.
The research objective is to crash test a thrie-beam design of a guardrail on 1H:1V slope under
MASH TL-3 test conditions (3-11 and 3-10). The thrie-beam system is envisioned to be aligned
with the slope break point. The posts are 9-ft long in the sloped section as shown in Figure 8.
Overall system layout is shown in Figure 9.
The envisioned research outcome is a MASH compliant system to replace the non-tested and
non-standard, inadequate systems, in place in locations that have restricted roadside.
A guardrail system in which the face of the rail is aligned with the slope break will provide
significant increase in shoulder width in mountainous areas as well as other locations that have
very restrictive space. This increased shoulder width will reduce nuisance hits while providing
TTI researchers will provide a report documenting the crash tests and their results per MASH
conditions along with videos and photographs.
Successful outcome of the crash tests of the 1H:1V guardrail system will be easily implemented
given an FHWA eligibility letter that will be issued for states that require such a letter.
Existing highways that pass through mountainous terrain may or may not have barrier systems to
protect errant vehicles. Even if there is a guardrail system is installed, its crashworthiness is not
established if it was on a steeper slope such as 1H:1V. A guardrail system in which the face of
the rail is aligned with the slope break will provide significant savings in shoulder width in
mountainous areas as well as other locations that have very restrictive space.
TTI will perform computer simulation of the thrie-beam system with and without rubrail to
identify the critical impact point (CIP) for each system for both MASH TL 3-10 and 3-11 tests.
TTI researcher will keep communicating with the state technical representative to choose the
candidate system for the crash testing Task.
TTI proving ground staff will construct the first test installation for the selected system
configuration from Task 1.
TTI will conduct the first MASH TL-3 test and upon the success of the first test, TTI proving
ground shall rebuild the test installation and conduct the second MASH test. The sequence of
which test shall be conducted first, 3-10 or 3-11 will be finalized during the early part of the
TTI researchers will summarize the results from Tasks 1, 2, and 3 and prepare a document
summarizing the crash tests results. TTI will prepare documentation of both tests and help
develop the content needed for the request of eligibility letter if both tests pass the MASH 2016
|TTI Research Supervisor:|
Texas A&M Transportation Institute
Texas A&M University System
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:|
Ted J. Whitmore, PE
Traffic Services Engineer
Traffic Engineering Division
1900 Kanawha Boulevard East
Charleston, WV 25305