D. Bullard

WASHINGTON STATE CABLE MEDIAN BARRIER

In the mid-1960s, the New York State Department of Transportation developed a three-strand cable barrier that has several desirable characteristics as compared with other roadside barriers. This system was crash tested in accordance with the NCHRP Report 350 crash test criteria as a roadside barrier. In the roadside barrier, all three cables are placed on the traffic side of the posts. Since 1988, the AASHTO Roadside Design Guide has contained information on a cable median barrier design in which the middle cable is mounted on the opposite side of the posts so that it can contain and redirect vehicles that strike the system from either side. In the early 1990s, the Washington State Department of Transportation (WSDOT) became interested in using this design for medians that are more than 10 m wide. WSDOT sponsored crash tests to evaluate the performance of this barrier, in accordance with NCHRP Report 350 criteria, with a small car and a pickup truck. In both tests, the vehicle was contained and brought to a stop. The occupant risk values were within the preferred limits set by NCHRP Report 350, and the damage to both vehicles was relatively minor.

Cable and Wire Rope Barrier Design Considerations: Review

Cable or wire rope barrier was being used in the 1940s and maybe earlier for vehicle containment. Through the years the designs have changed, but engineers continue to see cable barrier as an inexpensive barrier for use in some roadside applications. Recently, cable or wire rope has gained popularity as a median barrier for the prevention of cross-median accidents. Cross-median accidents are typically violent collisions with a high probability of multiple serious injuries and deaths. Thus, the design trend is gravitating toward providing positive vehicle containment in wider medians for which barriers have not historically been warranted. Wire rope often provides a cost-effective solution for this design scenario. Field experience with cable or wire rope barriers has identified areas for design improvement. It is desirable that cables remain taut to improve interaction with the vehicle, reduce dynamic deflections, and minimize maintenance. Additionally, reduced design deflections result in more potential application sites. Recent research demonstrates that such improvements are practical and cost-effective. Besides the initial tension in the wire ropes, other factors that can have a significant influence on dynamic deflections include post spacing and horizontal curvature. Computer simulations with cable barriers with various post spacings and horizontal curvatures were used to develop guidelines for expected design deflections. Finally, full-scale crash tests were completed with a new, cost-effective cable terminal system, and a brief review of the design and crash test results is included.

DEVELOPMENT AND TESTING OF STEEL U-CHANNEL SLIP SAFE SIGN SUPPORT

Researchers at Texas Transportation Institute designed a Slip Safe connection for use with steel U-channel sign-support structures within the highway right-of-way. The Slip Safe connection has two design configurations: a small Slip Safe design for single U-channel posts and a larger Slip Safe for back-to-back U-channel posts. Both configurations utilize Rib-Bak U-channel posts weighing up to 5.9 kg/m manufactured from 552-MPa yield billet steel. The Slip Safe is designed for use in a range of soil types with up to three supports per installation. There are two advantages of this Slip Safe design. First, the Slip Safe eliminates the dependency on material properties to achieve acceptable breakaway performance. In the past, all direct-driven and the majority of current breakaway designs for steel U-channel, square tube, wood, and round pipe sign supports have depended upon this. Second, the Slip Safe is omnidirectional and both hardware and supports are highly reusable due to the low energy required to activate the system. The Slip Safe designs are unlike most types of highway safety appurtenances in which performance is critically dependent on the failure of a bolt or bolts, the support material properties, or a combination of both. The Slip Safe has two advantages over other current U-channel connections. First, activation of the Slip Safe occurs with approximately one-third of the force needed to activate fracture-type connections. Second, the only disposable component in the Slip Safe connection is the keeper plate. All casting, bolts, and U-channel supports were undamaged, yielding economic advantages in high-frequency impact locations.

TESTING AND EVALUATION OF MERRITT PARKWAY GUIDERAIL

A blocked-out steel-backed timber guiderail and transition on steel posts were successfully developed and crash-tested in accordance with requirements set forth in NCHRP Report 350 for Test Level 3. The timber guiderail system was developed for placement along the scenic Merritt Parkway in Connecticut. The timber guiderail and transition were constructed from rough-sawn commercial Grade 1 southern pine backed with a 10- x 152-mm ASTM A588 Grade 50 structural steel plate. The guiderail was supported by W150 x 22.5 steel posts 2 m long, spaced 3.05 m through the length-of-need section. Reduced post spacings of 0.8 m and 1.5 m were used in the transition section.

NCHRP Report 350 compliance tests of the ET-2000

The ET-2000 is one of the end treatments currently approved for use with W-beam guardrail systems. The ET-2000 has successfully met all evaluation criteria set forth in NCHRP Report 230. However, with the adoption of NCHRP Report 350 by FHWA as the official guidelines for crash testing of roadside safety features, it became necessary to reevaluate the ET-2000 to the new guidelines. It is noted that one of the design test vehicles specified in NCHRP Report 230, the 2044-kg passenger car, was replaced by a 2000-kg pickup truck (2000P) under NCHRP Report 350 guidelines. The purpose of the crash tests was to evaluate the ET-2000 according to NCHRP Report 350 guidelines. The ET-2000 met NCHRP Report 350 criteria for Performance Level 3 without any design modifications. All findings in this study demonstrate that the impact performance of the ET-2000 was satisfactory.

Reusable High-Molecular-Weight, High-Density Polyethylene Crash Cushions for Wide Hazards

The development and full-scale crash testing of a new, reusable, and essentially maintenance-free crash cushion designed to shield wide hazards are described. The energy-dissipating medium in this device is composed of high-molecular-weight, high-density polyethylene cylinders. This “smart” energy-dissipating thermoplastic is the same material used in the REACT family of narrow-hazard crash cushions and truck-mounted attenuators. It is self-restorative and reusable and possesses excellent energy-dissipation properties. The design process incorporated the results of impact testing with individual polyethylene cylinders, finite-element modeling, and a comprehensive full-scale crash testing program conducted in accordance with the guidelines of NCHRP Report 350. It is demonstrated that finite-element modeling is a cost-effective tool whose use can minimize the number of costly full-scale crash tests required to develop an effective impact attenuation device. This new, reusable, wide-hazard crash cushion satisfies all of the requirements of NCHRP Report 350 and has been accepted by FHWA for use in the National Highway System.