Three cable guardrail is commonly used to protect motorists from roadside slopes. However, concern
arises when the barrier must be placed close to a steep slope. Previously, the Midwest States Pooled
Fund Program funded research project to investigate the performance of low-tension cable barrier
adjacent to slopes as steep as 1.5H:1V. Researchers at the Midwest Roadside Safety Facility (MwRSF)
conducted a test, test no. CS-1, with a Â¾-ton pickup truck on a standard, three cable guardrail set back
12 in. from the slope break point of a 1.5:1 slope. The system consisted of 63-in. long, S3x5.7 line posts
spaced 16 ft on center. During the Test Level 3 (TL-3) 2000P crash test, the posts rotated without much
soil resistance, resulting in the vehicle becoming completely airborne and encroaching onto the steep
slope. As a result, the front-impact side of the vehicle dropped below the vehicle c.g., thus causing the redirective
forces applied by the cable system to be significantly below the c.g. of the vehicle. This induced
a â€œtrippingâ€ effect and applied a roll moment on the vehicle, causing the vehicle to roll over the cables and
come to rest at the bottom of the embankments. Thus, the standard, three cable guardrail system set
back 12 in. from the slope break point of a 1.5:1 slope performed unsatisfactorily according to the TL-3
safety performance criteria in NCHRP Report No. 350.
The poor performance observed in test no. CS-1 warranted design modifications. Consequently,
additional computer simulation with BARRIER VII predicted that a reduced post spacing of 4 ft showed an
increase in the lateral barrier stiffness and consequently reduced lateral barrier deflections. The postspacing
reduction was coupled with an increased barrier offset from the slope breakpoint. Increasing the
offset to 4 ft would limit the penetration of the vehicle onto the slope and reduce the potential vehicle c.g.
drop. It is believed that these design changes will significantly improve the safety performance of the
standard three-cable guardrail system when installed adjacent to a 1.5H:1V slope. A second full-scale
crash test, test no. CS-2, found that implementing these changes resulted in a safe redirection of a 2000P
vehicle according to TL-3 of NCHRP Report No. 350.
While the design modifications implemented for test no. CS-2 provided for safe redirection of vehicles
when the cable system was placed adjacent to steep slopes, there were some drawbacks. The closely
spaced posts used in the modified system can be difficult and costly to install. The additional offset from
the slope break point can also be difficult to achieve in practice. Thus, a need exists to reconsider the
design of low-tension cable barrier adjacent to steep slopes to alleviate these issues.
The objective of this proposed study is to review the design of the low-tension cable barrier adjacent to a
steep slope and determine improved design options that would increase post spacing and reduce the
offset of the barrier from the slope. If design modifications for improving the cable barrier adjacent to
slope were developed, full-scale crash testing according to the TL-3 criteria in MASH would be used to
evaluate the system.
Redesign of the low-tension cable barrier adjacent to steep slopes would provide end users with a lower
cost and more easily installed solution for shielding steep slopes that would still provide safe redirection of