Roger P Bligh

Developing Guidelines for Median Barrier Installation: Benefit-Cost Analysis with Texas Data

Guidelines for the installation of median barriers presented in the AASHTO Roadside Design Guide have remained essentially unchanged for more than 30 years. In recent years, the need for improved guidance has prompted several states to reevaluate their guidelines and has also precipitated a nationwide research project administered by the Transportation Research Board. The objective of the study, on which this paper is based, was to develop improved guidelines for the use of median barriers on new and existing high-speed, multilane, divided highways in Texas. The purpose here is to present some modeling and benefit–cost analysis results from that study, with a focus on the results from a particular data set developed under a cross-sectional with–without study design. The highways of interest are those classified as Interstates, freeways, and expressways with four or more lanes and posted speed limits of 55 mph (88 km/h) or higher. The models employed to estimate median-related crash frequencies and severities, including the Poisson-gamma and ordered multinomial logit models as well as modeling results from a full Bayes estimation method, are presented. From the modeling results, a preliminary benefit–cost analysis is described, in conjunction with some sensitivity analyses, for developing the guidelines for concrete and high-tension-cable barriers. A discussion of the limitations of this study and potential future extensions is provided.

Part 1: Roadside Safety Design: Developing Guidelines for Median Barrier Installation: Benefit-Cost Analysis with Texas Data

Guidelines for the installation of median barriers presented in the AASHTO Roadside Design Guide have remained essentially unchanged for more than 30 years. In recent years, the need for improved guidance has prompted several states to reevaluate their guidelines and has also precipitated a nationwide research project administered by the Transportation Research Board. The objective of the study, on which this paper is based, was to develop improved guidelines for the use of median barriers on new and existing high-speed, multilane, divided highways in Texas. The purpose here is to present some modeling and benefit-cost analysis results from that study, with a focus on the results from a particular data set developed under a cross-sectional with-without study design. The highways of interest are those classified as Interstates, freeways, and expressways with four or more lanes and posted speed limits of 55 mph (88 km/h) or higher. The models employed to estimate median-related crash frequencies and severit...

Performance limit analysis for common roadside and median barriers using LS-DYNA

Performance limits of commonly used barriers in terms of acceptable vehicle impact heights were identified using non-linear finite element methodology. Finite element models for four widely used guardrail systems were developed. The selected guardrail/barrier systems are the modified G4(1S) W-beam guardrail, modified thrie-beam guardrail, Midwest guardrail system, and modified weak post W-beam guardrail. LS-DYNA simulations using these guardrail models were validated based on the results obtained from existing crash tests performed on flat terrain. To define vehicle containment limits, LS-DYNA simulations were performed with variable vehicle-to-barrier impact heights on guardrails placed on flat terrain. Override and underride limits for each guardrail model were identified using NCHRP Report 350 design vehicles. Vehicle impact height was parametrically varied to determine performance limits of each barrier as defined by initiation of override or rollover for the pickup truck and underride for the small passenger car.

Crashworthiness of roadside features across vehicle platforms

Each year more than 14,000 persons are killed and 1 million persons are injured as a result of roadside crashes. Recent estimates of the annual societal costs associated with these roadside crashes amounted to

CRASH TESTING AND EVALUATION OF WORK ZONE BARRICADES

80 billion. Clearly, there is a need for further understanding of the roadside safety problem. The impact performances of roadside safety features are typically evaluated through full-scale crash testing with two vehicles selected from the extremes of the passenger vehicle fleet in terms of weight and size. The implicit assumption is that if a roadside safety feature successfully passes the test requirements for vehicles at the extremes for the fleet, the feature will perform satisfactorily for all other vehicles in between. Many vehicle parameters could influence performance during impacts, and this assumption may or may not be valid. The safety performances of roadside features for various passenger car platforms and light-truck subclasses were evaluated. The study approach consists of evaluations of the frequency and severity of roadside crashes for these generic platforms and subclasses by using recent crash data from the Fatal Accident Reporting System, the General Estimates System, and the Highway Safety Information System.

Crash Testing and Evaluation of Work Zone Traffic Control Devices

Proper traffic control and delineation are critical to achieving safety in work zones. However, the work zone traffic control devices themselves may pose a safety hazard to vehicle occupants or work crews when impacted by errant vehicles. Thus, there was a need to research the safety performance of work zone traffic control devices to ensure that they perform satisfactorily and meet NCHRP Report 350 guidelines. Several research studies sponsored by the Texas Department of Transportation evaluated the impact performance of various work zone traffic control devices, such as temporary and portable sign supports, plastic drums, sign substrates for use with plastic drums, traffic cones, and vertical panels. Specifically addressed are the studies on barricades. Standard wooden barricade construction was found to be unacceptable due to a demonstrated potential for intrusion of fractured members into the occupant compartment. In response to deficiencies identified in the wooden barricade tests, several alternate barricade designs were developed and successfully tested.

Assessment of NCHRP Report 350 Test conditions

Safety of work zones is a major area of concern since it is not always possible to maintain a level of safety comparable to that of a normal highway not under construction. Proper traffic control is critical to the safety of work zones. However, traffic control devices themselves may pose a safety hazard when impacted by errant vehicles. The impact performance of many work zone traffic control devices is mostly unknown, and little, if any, crash testing has been conducted in accordance with guidelines set forth in NCHRP Report 350. The Texas Department of Transportation (TxDOT) has, in recent years, sponsored a number of studies at the Texas Transportation Institute to assess the impact performance of various work zone traffic control devices, including plastic drums and sign substrates, temporary and portable sign supports, plastic cones, vertical panels, and barricades. The results, findings, conclusions, and recommendations are presented for temporary and portable sign supports, plastic drums, sign substrates for use with plastic drums, traffic cones, and vertical panels, whereas those for barricades are covered elsewhere. Most of the work zone traffic control devices satisfactorily met the evaluation criteria set forth in NCHRP Report 350 and are recommended for field implementation. However, some of the devices failed to perform satisfactorily and are not recommended for field applications. The results from these studies are being incorporated into the TxDOT barricade and construction standard sheets for use in work zones.

Low-Deflection Portable Concrete Barrier

The appropriateness of test conditions specified in NCHRP Report 350 was assessed. The assessment focused on the basic Test Level 3 and addressed two areas of interest: the effects of higher speed limit on impact speed and the appropriateness of 25° for the impact angle. The following conclusions and recommendations were drawn on the basis of the results of the analysis: (a) the current test impact speed of 100 km/h (62.2 mph) should be maintained, (b) the current impact angle of 25° for Test 11 of length-of-need sections of permanent longitudinal barriers should be maintained, (c) the test impact angle should be reduced from 25° to 20° for Test 11 of length-of-need sections of temporary longitudinal barriers, and (d) the test impact angle should be reduced from 25° to 20° for Test 21 of barrier transition sections. However, the selection of impact conditions is more a policy decision than a technical issue to be resolved in the update of NCHRP Report 350 guidelines.

Cable and Wire Rope Barrier Design Considerations: Review

Temporary barriers are often required to provide positive protection for motorists and workers in a highway work zone. Most highway work zones are restricted in regard to available lateral space for accommodating traffic and the work activity. Consequently, it is desirable to minimize deflection of work zone barriers to minimize the required buffer distance between the barrier and work activity area and, thereby, maximize the space and number of lanes available for traffic. Under this study, a new connection designed to reduce dynamic deflection of portable concrete traffic barriers was developed through a program of finite element simulation and full-scale crash testing. The new cross-bolted (or X-bolt) connection uses two threaded rods in different horizontal planes across the barrier joint to form a tight, moment connection. It achieves the objective of low dynamic barrier design deflection without sacrificing constructability. In addition to being easy to install, the new barrier system is also perceived to be easy to inspect and repair. Crashworthiness and design deflection of the barrier connection were verified through full-scale crash testing using segment lengths of 10 ft and 30 ft. An F-shape barrier with X-bolt connection was demonstrated to have the lowest deflection of any approved portable concrete barrier.

Placement of traffic barriers on roadside and median slopes - guidelines based on numerical simulations

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.