United States Department of Transportation
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Abstract:Road barriers have been used as an effective countermeasure to prevent exposure of errant vehicles to both vehicles travelling in the opposite direction
and to roadside hazards. The objective of this study was to evaluate the in-service safety performance of three types of road barriers (concrete barriers,
steel W-beam guardrails, and high-tension cable barriers) in Indiana using cross-sectional analysis based on crash data. The quantitative evaluation was
comprised of three components: 1) the effect of the road, barrier scenarios, and traffic on the barrier-relevant (BR) crash frequency, 2) the effect of the
road and the barrier scenarios on the BR harmful events, and 3) the effect of the BR events and other conditions on the injury outcomes.
The introduction of the BR harmful events linked the crash onset with its outcome. The three developed statistical models were connected through their
inputs-outputs and followed the sequence of various BR events during the BR crash. This improvement allowed a more comprehensive and insightful
analysis of the barriers’ safety effects and a more efficient use of data. The injury outcomes were estimated for all the individuals in a crash rather than for
the most severe outcome of a crash. Further improvement of the cost estimates was accomplished by utilizing hospital data.
For median barriers, this study found that the total number of BR crashes was higher with the use of median barriers, mostly due to the introduction of
collisions with barriers and an increase in the collisions after redirecting vehicles back to traffic. These undesirable effects of barriers were surpassed by
the positive results of reducing hazardous events such as cross-median crashes, rollover events, and collisions with roadside hazards, which substantially
reduced the number of severe injuries and fatalities.
The average (unit) crash costs were estimated for roads without barriers and for roads with various barrier scenarios. The crash costs were reduced by
50% where cable barriers were in medians wider than 50 feet and where concrete barriers or guardrails were in medians less than or equal to 50 feet
wide. Roadside barriers (guardrails) reduced the unit crash costs by 20% to 30%.
Median cable barriers were found to be the most effective among all the studied barriers due to the smallest increase in the crash frequency and least
severe injuries in barrier-relevant crashes. A cable barrier’s offset to the travelled way was also investigated in this study. When considering vehicles
moving in one direction, the nearside cable barriers installed at an offset less than or equal to 30 feet performed better than far-side cable barriers with a
larger offsets thanks to the better protection they provide for vehicles against rollovers in the median and impact with the median drain. Consequently,
the biggest safety benefit can be expected where cables barriers are installed in the median at both edges.
The results were implemented through a set of crash modification factors and unit crash costs estimated for 51 road-barrier scenarios. An implementation
procedure is provided to quantify the crash costs and the safety benefits for these scenarios.
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