Impact Performance Assessment of Barrier Performance at High Speeds
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).
This project will determine
impact conditions and barrier designs appropriate for use on roadways with
posted speeds of 75 mph and greater. The
appropriate impact conditions for roadways with higher speed limits (75 mph and
greater) through crash data analysis,
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.
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.
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.
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
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.
NCHRP Project 17-43, “Long-Term Roadside Crash Data Collection
Program,” Transportation Research Board (Active).
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.
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).
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.
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.
NCHRP Project 17-79, “Safety Effects of Raising Speed Limits to 75 mph
and Higher,” Transportation Research Board (Active).
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.
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).
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|
|RNS Developer:||Alex Price, Chiara Silvestri Dobrovolny, and Roger Bligh|
|Source Info:||2018 AFB20 Midyear Meeting, Austin, TX|
|Index Terms:||Barriers (Roads), Speed, Crashworthiness, Impact, Highway safety, |
Safety and Human Factors