Road safety fences are increasingly becoming important. Crash barriers are used as a longitudinal protective system to reduce the severity of accidents.
It is site constraints which play a major role in the selection and placement of the guardrail. Based on the location, these constraints should examine the possibilities in including environmental considerations, topographic challenges, restricted right-of-way, geological concerns or conflict with other infrastructure if required.
The decision to install a crash barrier is made on the basis of a warrant analysis. Typically, an analysis of certain elements of the road conditions within a clear zone determines the preference. This judgement, in turn depends on the engineer’s professional experience supported by evidence of research findings.
Warrants for barriers are established on the basis of cost benefit analysis. Hazards that may warrant shielding by barriers include embankments and other road side obstacles. It is therefore imperative that while determining the need to install such a barrier, the basic factors such as embankment, embankment height, side slope, encroachments and other geographical constraints are thoroughly examined.
Similarly, a median barrier would prevent an errant vehicle crossing the median on a divided road section of the highway and heading into oncoming traffic. In this case, the median width, the average daily traffic and terrain characterises are the primary factors for determining need. Warrants for this kind of a barrier have been established on the basis of an evaluation of median cross over accidents.
The ‘Median Barrier Warrant Guide’ produced by the Ontario Roadside Safety Manual clearly recommends that there is no need to provide median barriers if the width of the median exceeds 15 metres. For widths between 10 and 15 metres, installation is optional. Based on experience, it is felt that a number of locations characterised by an entrance ramp from the arterial cross slopes with a combination of embankment height, ramp curvature and embankment slope become potentially hazardous.
Such protective installations can be divided into main groups: Edge barriers (or guard rails) and crash barriers. An Edge barrier, also known as a roadside barrier, as mentioned above, is generally placed at the edge of the road whereas the crash or median barrier is installed in the median of a divided road.
A crash barrier located within a narrow central reservation is generally designed to prevent vehicles from crossing into the opposing stream. However, longitudinal fences can themselves become a hazard to vehicles.
They are therefore required to be designed keeping in view that if a vehicle hits a safety fence, it will result in either avoidance of road accidents or a reduction in the severity of the accident. The majority of longitudinal safety fences available commercially can be classified as i) Metal Beam (Semi-Rigid), ii) Rigid Concrete and iii) flexible cable. These same classifications are used for edge and median barriers.
Crash barriers are also found to be of immense use as bridge barriers, work zone barriers and temporary barriers.
In order to reduce the severity of road accidents crash attenuation devices are increasing being used to absorb the impact. When there is a sudden change from one barrier type to a more rigid barrier type, a vehicle striking the flexible barrier may be intercepted in the deflected barrier pocket and directed into the more rigid barrier. This is commonly referred to as “pocketing.” A transition stiffens the more flexible barrier by decreasing the post spacing, increasing the post size and using stiffer beam elements.
While evolving a mechanism for safe and efficient roads, several parameters are taken into account. These include i) design criteria, ii) assessment of effectiveness road side infrastructure including crash barriers, iii) case studies/examples along with its best practices.