Where roadside hazards such as embankments and fixed objects cannot be removed, barriers are used to reduce the severity of run-off-road (ROR) crashes. Median barriers are also used to prevent head-on crashes resulting from right-side lane departures.
Types of barriers
Road safety barriers consist of several main types, including:
Flexible barriers (e.g., wire rope) offer excellent safety performance for light vehicles through high dynamic deflection and low impact forces, resulting in lower crash severities. While wire rope barriers have greater flexibility, W-beam systems with light flexible posts may also meet design objectives. Flexible barriers are less expensive but require more frequent maintenance.
Semi-rigid barriers (e.g., W-beam) provide moderate dynamic deflection to reduce impact forces, combined with a steel post and W-beam design that helps contain vehicles and prevent post-impact rollovers. With less deflection than wire rope, these barriers are suitable if clearance to roadside objects is limited.
Rigid barriers (e.g., concrete) offer little or no dynamic deflection when impacted. As such, these are more effective for heavy vehicle containment than flexible and semi-rigid barriers. However, high impact forces may increase injury severity and induce more vehicle rollovers compared with other barrier types. Impact forces are somewhat reduced with sloped barrier faces. Rigid barriers are relatively costly but have lower maintenance requirements.
Effectiveness
The Austroads Guide to Road Safety Part 2 documents the effectiveness of the three barrier types from a 2016 Australian study. Reductions in ROR fatal and serious injury crashes were 83%, 74% and 12% for wire rope, W-beam and concrete barriers, respectively.
Considerations
Ideally, permanent barriers should be considered as a proactive ‘continuous treatment’ approach, rather than to treat specific or isolated hazards. Different barrier types may connect in a continuous treatment as appropriate for specific road sections.
Barrier selection should clearly consider the full range of relevant traffic and environmental factors, including vehicle types, as well as installation and ongoing maintenance costs. Posts in flexible and semi-rigid barriers can be hazardous for motorcyclists, some of which can be addressed with underrun protection.
