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Impact Performance Assessment of Barrier Performance at High Speeds

Description:

Recent data indicates that sixteen (16) states have speed limits of 75 mph or higher. Seven (7) of these states currently have speed limits of 80 mph, with almost all of them adopting this higher speed within the last 5 years.

The AASHTO Manual for Assessing Safety Hardware (MASH) specifies impact speeds for crash testing and evaluation of barrier systems such as guardrails, median barriers, and bridge rails. The highest impact speed defined by MASH for passenger vehicle is 62 mph. This impact speed was derived from analyses of reconstructed crash data that is now nearly 20 years old. Preliminary data from NCHRP Project 17-43, under which a new database of reconstructed run-off-road crashes is being developed, indicates that for highways with a posted speed limit of 70 mph or greater, the 85th percentile impact speed is (1). This indicates a need for a higher design impact speed for barriers used on these higher speed roadways.

A recent study of single vehicle run-off-road crashes in Texas found that as the posted speed limit increases from 70 mph to ≥ 75 mph, the rate of severe injuries and fatalities increases (2). With more states adopting higher speed limits and posting an increasing number of miles of roads with speed limits of 75 mph and higher, there is a growing recognition of the need for evaluating barriers for higher impact speeds in order to maintain the level of safety for motorists on these roadways. In one recent project, a bridge rail system developed and tested under MASH was subsequently subjected to supplemental crash testing with passenger vehicles at an impact speed of 75 mph.

Increased impact speeds will place more demand on barrier systems. The performance limits of current barrier systems beyond the MASH design impact speed of 62 mph are not fully understood. There have been observations of guardrail systems developing partial rail tears when tested under MASH impact conditions, indicating that these systems are near their containment capacity (3,4). There is an urgent need to assess performance limits of existing barrier systems for higher impact speeds and to provide user agencies with barrier systems appropriate for use on roadways with higher posted speeds (> 70 mph).

Objective:

This project will determine impact conditions and barrier designs appropriate for use on roadways with posted speeds of 75 mph and greater. The objectives include:

· Determine appropriate impact conditions for roadways with higher speed limits (75 mph and greater) through crash data analysis,

· Assess performance limits of existing barrier systems using engineering analysis and finite element simulation, and

· Develop and evaluate new or modified barrier systems capable of accommodating the recommended design impact conditions using finite element simulation and full-scale vehicle crash testing.

Benefits:

Sixteen (16) states have increased permissible speed limits to 75 mph or greater. Seven (7) of these states currently permit posted speed limits of 80 mph, with all but one of these stated implementing this change within the last 5 years. Recent research indicates that roadways with higher posted speeds (≥ 75 mph) have a higher rate of severe injury and fatal crashes. Existing barrier systems are designed for impact speeds of 62 mph.

There is an urgent need to develop barrier systems appropriate for use on these higher speed roadways to help mitigate the severity of run-off-road crashes. In absence of this research, the frequency of severe injury and fatal crashes will likely escalate as more miles of roadway are posted with higher speeds and vehicle miles of travel continue to increase.

Related Research:

The AASHTO MASH document specifies impact speeds that are used for crash testing. The recommended passenger vehicle impact speed of 62 mph was derived from analyses of reconstructed run-off-road crash data collected under NCHRP Project 17-22. This database relied on NASS CDS data from 1997 to 2001 along with supplemental onsite roadway and roadside data collection. The study found that 62 mph was the 85th percentile impact speed (5).

NCHRP Project 17-43 was funded to update the NCHRP Project 17-22 database and develop a long-term crash data collection program to aid in analysis of run-off-road crashes (1). Preliminary data from NCHRP Project 17-43 indicates that highways with a posted speed limit of 70 mph and greater have an 85th percentile impact speed of 77 mph. This indicates a need for evaluation of barriers at higher impact speeds for roadways with higher posted speeds.

NCHRP Project 17-79 was funded to investigate safety effects of raising speed limits to 75 mph and higher, and provide guidance to assist highway agencies in estimating the safety impacts of increasing speed limits (6). However, the NCHRP Project 17-79 scope is limited to studying the effects of increased speed limits on crash frequency and severity through crash data collection and analysis. There is no determination of recommended design impact speeds for evaluation of barrier systems intended for use on roadways with higher speed limits and no direct assessment of barrier performance at these higher impact speeds through computer simulation or full-scale testing.

The Texas Department of Transportation (TxDOT) funded Project 0-6771 to evaluate barriers for use on high speed roadways with posted speed limits up to 85 mph. Based on extrapolation of impacts speeds from the NCHRP Project 17-22 database, the 85th percentile impact speed was estimated to be 68 mph. As noted above, this database relies on crash data collected between 1997 and 2001, which predates the increase in posted speed limits in many states. Computer simulation was used to evaluate the impact performance of selected barrier systems at an impact speed of 68 mph. The predictive results suggest that some current barrier systems have a reasonable chance of acceptable impact performance at this higher speed. No full-scale crash testing was performed to verify impact performance.

References

  1. NCHRP Project 17-43, “Long-Term Roadside Crash Data Collection Program,” Transportation Research Board (Active).

  2. Xavier, C., “Evaluating the Relevancy of Crash Test Guidelines for Roadside Safety Barriers Placed on High Speed Roads,” Undergraduate Research Scholars Thesis, Texas A&M University, College Station, Texas, 2015.

  3. Silvestri Dobrovolny C., “MASH Full-Scale Crash Testing of a 31-inch Buried-in-Backslope Terminal Compatible with an MGS Guardrail System,” Roadside Safety Pooled Fund Group, (Active).

  4. Lechtenberg, K., “Recent MASH Implementation Testing at MwRSF,” Presentation at the AFB20 Roadside Safety Design Committee, 97th Annual Meeting of the Transportation Research Board, Washington D.C., 2018.

  5. Mak, K., Sicking, D., and Coon, B. “NCHRP Report 665. Identification of Vehicular Impact Conditions Associated with Serious Ran-off-Road Crashes,” Transportation Research Board, Washington D.C., 2010.

  6. NCHRP Project 17-79, “Safety Effects of Raising Speed Limits to 75 mph and Higher,” Transportation Research Board (Active).

  7. Silvestri Dobrovolny C., Sheikh, N., Bligh, R., White, K., “Development and Finite Element Analysis of Longitudinal Barrier Systems for Use on High-Speed Roadways,” Report No. 0-6771-1, Texas Transportation Institute, College Station, Texas, 2013.

Implementation:

The result of this research project will be crashworthy barrier systems suitable for use on higher speed roadways with posted speed limits of 75 mph and higher. Successfully tested barrier systems will be immediately available for implementation by State DOTs on roadways with higher posted speeds. The barrier systems could be further implemented through an update of the appropriate sections of the AASHTO Roadside Design Guide (RDG). Recommended design impact conditions could be incorporated into a future update of MASH with proper review and consideration by AASHTO Technical Committee on Roadside Safety (TCRS).

Results of this research and the resulting impact conditions and barrier designs will be disseminated to state DOT design engineers, consultants, and practitioners through presentations and webinars at appropriate industry and professional meetings including TRB Roadside Safety Design Committee, AASHTO TCRS, and Committee on Bridges and Structures Technical Committee T-7 “Guardrails and Bridge Rails.”

Sponsoring Committee:AFB20, Roadside Safety Design
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Alex Price, Chiara Silvestri Dobrovolny, and Roger Bligh
Source Info:2018 AFB20 Midyear Meeting, Austin, TX
Date Posted:01/15/2019
Date Modified:01/25/2019
Index Terms:Barriers (Roads), Speed, Crashworthiness, Impact, Highway safety,
Cosponsoring Committees: 
Subjects    
Highways
Design
Safety and Human Factors

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