Rigid central safety barriers in constrained road environment

Holzner, Katherine (2014) Rigid central safety barriers in constrained road environment. [USQ Project]


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Road crashes are a continuing source of personal grief and economic loss in most societies. In NSW between October 2008 and September 2013, head-on crashes made up 5.5% of all rural road rashes in NSW but contributed to 27% of all fatalities on those roads.
Head-on crashes tend to be concentrated at sites of substandard horizontal road geometry. Often, rectification of site geometry on constrained rural roads is beyond the limitations of Road Authorities. Installation of central barriers in narrow medians is a cost effective incremental solution but often comes at a sacrifice – reduction of sight distance, reduced recovery area and surface drainage implications.
This study assesses the implication of introducing a concrete central safety barrier (CCSB) into an already substandard, complex road environment by assessing the before-and-after performance of nine concrete central safety barrier sites.
The literature review compared the different types of central safety barriers including concrete (cast in-situ and pre-cast), guardrail and wire rope. When compared to wire rope and guardrail, concrete barriers are more suited to highly constrained road environments, having negligible deflection, being suitable for horizontal radius less than 200m and remaining operational post collision (reduced maintenance and worker exposure). The compromise with the system is increased crash severity, highest potential implications on surface drainage and sight.
Although the severity index of concrete barriers is seemingly the highest of all barrier systems, the results typically agreed with general performance of all barriers: non-injury (tow-away) crashes increase in place of injury crashes and injury crashes increase in place of fatal crashes. Fatal crashes were almost entirely eliminated.
CCSB were found to be an effective solution at eliminating head-on crashes at all nine sites investigated. The installation also had an improvement of general road safety at seven of the nine sites analysed. It is believed that the poor performance of the two remaining sites was likely attributed to poor co-ordination of horizontal and vertical geometry at one site and worsening of surface drainage conditions at the other.

The total average reduction in fatalities across all sites was 111% and factored severity costs in all accidents reduced by 200%.
Six of the nine sites had pre-cast concrete central barrier systems. Currently there is no current acceptance for the use of precast concrete barriers as a permanent installation. All six pre-cast sites had an overall positive impact on road safety, it is recommended this system be tested for acceptance as an RMS approved barrier system.
Stopping sight distance (SSD) is the distance required to enable a driver to perceive, react and break to a stop before reaching a hazard on the road ahead. SSD is frequently viewed as an overriding parameter which directly relates to road safety within the road design community. The literature review revealed that the SSD model was based on a number of 85th percentiles combined with a small hazard being on the roadway. This has created quite a conservative design parameter which is often difficult to achieve, especially in constrained environments. The model may also stretch the limits of human abilities.
The significant reduction of SSD at the sites, ranging from achieving 33-100% of the required SSD value, did not directly result in significant increase in crash severity. There was no distinct relationship between the degree of SSD reduction and increase in crashes.
A less conservative, more realistic model such as ‘SSD over barriers’ is more suited to the constrained road environment. This model sights to a vehicle brake light or top of car and requires the provision of a 2.5m lane for manoeuvring around the object. However, as most sites failed to meet this model, no link between meeting ‘SSD over barrier’ and the reduction in crash severity was observed.
Consequential poor pavement drainage at one site was likely to have attributed to a significant increase in wet pavement crashes. This is likely related to the cast in-situ concrete barrier which had only small drainage slots. The slots were not likely to relieve adequate pavement flows and may have led to aquaplaning.
A site on the Princes Highway at East Lynne has been selected to apply the results and conclusions drawn from this study.

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Item Type: USQ Project
Item Status: Live Archive
Additional Information: Civil Engineering and Surveying project.
Faculty/School / Institute/Centre: Historic - Faculty of Health, Engineering and Sciences - School of Civil Engineering and Surveying (1 Jul 2013 - 31 Dec 2021)
Supervisors: Ayers, Ron; Ellis, Peter
Date Deposited: 09 Sep 2015 04:53
Last Modified: 03 Mar 2016 04:59
Uncontrolled Keywords: road safety; central safety barriers; sight distance; road crashes; New South wales; NSW; head-on crashes; fatalities; substandard horizontal road geometry; central barriers; concrete central safety barriers; CCSB; guard rail; wire rope; sight distance; collisions; stopping sight distance; SSD; Princes Highway; East Lynne
Fields of Research (2008): 09 Engineering > 0912 Materials Engineering > 091299 Materials Engineering not elsewhere classified
12 Built Environment and Design > 1204 Engineering Design > 120403 Engineering Design Methods
09 Engineering > 0905 Civil Engineering > 090507 Transport Engineering
09 Engineering > 0902 Automotive Engineering > 090204 Automotive Safety Engineering
Fields of Research (2020): 40 ENGINEERING > 4016 Materials engineering > 401699 Materials engineering not elsewhere classified
40 ENGINEERING > 4010 Engineering practice and education > 401001 Engineering design
40 ENGINEERING > 4005 Civil engineering > 400512 Transport engineering
40 ENGINEERING > 4002 Automotive engineering > 400204 Automotive safety engineering
URI: https://sear.unisq.edu.au/id/eprint/27368

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