Safety barriers, or crash barriers, collectively known as vehicle restraint systems (VRS), are a highly visible element of our modern road network. There are many variations of the systems utilised and the choice of system is dependent on a number of factors including: location within the road cross section (i.e. verge or median), the nature of the hazard being protected, and other site-specific constraints such as the space available. VRS types include:

  • Flexible systems, such as tensioned wire rope;
  • Semi-rigid barriers which include corrugated or open box steel beam systems; and
  • Rigid (or concrete) barriers.
Bridge parapets are also classed as a vehicle restraint system. As indicated by the name, the function of a VRS is to restrain vehicles and contain them within the road space, preventing an errant vehicle from either crossing the central reservation, driving off a bridge or colliding with a hazard in the roadside verge. They are typically installed at verge-side locations, where the risk of injury to occupants of an errant vehicles is higher if the vehicle collides with a roadside hazard than if it collides with the safety barrier. Single-vehicle collision statistics on national roads for 2014 showed that there were 1,553 collisions in total with different roadside objects. Of the total number of collisions, some 60 were with trees and of these eight were fatal. There were 297 impacts with roadside steel barriers and none of these were fatal. Notwithstanding the risk of injury from impact with a roadside barrier, it is clear that safety barriers reduce the risk of serious injury. All VRS types are designed to serve a dual purpose. They cushion the impact of the collision by absorbing some of the kinetic energy of the vehicle through deformation of the barrier when impacted. Secondly, they are designed to avoid bringing the vehicle to a sudden stop, but instead to re-direct the vehicle back onto the road. The re-direction is achieved in a controlled manner, on a course as close as possible to parallel to the barrier so as to leave the vehicle in a safe position after the collision. When an errant vehicle strikes a safety barrier, the barrier should behave in a way that follows a pre-determined sequence, provided the design and installation have been completed correctly.

Barriers transfer impact energy

[caption id="attachment_36538" align="aligncenter" width="300"]CLICK TO ENLARGE Fig 1: VRS types commonly found on the national road network CLICK TO ENLARGE Fig 1: VRS types commonly found on the national road network[/caption] Taking the example of a semi-rigid roadside barrier, at impact the rail yields and deflects under contact pressure and transfers the impact energy to the surrounding posts. The posts bend at the base followed by the breaking or pulling out of the bolts connecting the post to the rail, causing the two to separate. This is a critical aspect of the barrier’s performance because it allows the posts to bend as the barrier deflects without pulling the longitudinal rail down. If detachment of the post and rail does not occur, the posts will bend and pull the rail down, potentially allowing the vehicle to travel over the barrier. After the vehicle impacts the rail, its trajectory is altered by the reaction of the barrier and it begins to slide along the length of the barrier causing further posts to bend and disconnect in a domino-like effect. As long as the detachment of the rail and each post takes place, and the barrier has been installed at the correct height, the vehicle will not breach the barrier but instead will continue to be re-directed back onto the road. While this is happening, the impact energy is dissipated through the deformation of the barrier system, the deformation of the vehicle itself and friction between the two. The amount of deflection of the barrier during impact has to be designed to avoid the vehicle penetrating to the hazard being protected. The amount of deflection plus the lateral width of the barrier is known as the working width and is a fundamental design parameter. The design of VRS differs from conventional structural design in that the structures are firstly designed to fail in a predictable fashion. VRS design is more about making the structure fail in a particular way rather than how to prevent failure, as it is the failure mechanism of the structure that provides protection to the driver. To add further complexity, the whole process from impact to when the vehicle comes to a halt lasts between 2-3 seconds and is governed by a number of factors. These include ground conditions, mechanical properties of the barrier material, the height of the barrier, the speed, type and angle of the impacting vehicle and the local topography.

Crash tests demonstrating VRS performance

[caption id="attachment_36534" align="alignright" width="300"]Fig 2: Deflection of steel VRS post impact Fig 2: Deflection of steel VRS post impact[/caption] Due to the above complexities, the only effective way to demonstrate the performance of vehicle restraint systems is to carry out full crash tests on the systems to see what actually happens upon impact under various test scenarios. Manufacturers design their systems to meet certain performance criteria under impact from different vehicle types and weight, speed and angles of impact. The tests are conducted in accordance with the harmonised European standards (IS EN1317) and the performance of the system must accord with the requirements of the harmonised standard in order to be allowed to be placed on the market in Ireland and across the EU. Under the provisions of the Roads Act, Transport Infrastructure Ireland (TII) is charged with responsibility “to secure the provision of a safe and efficient network of national roads” in Ireland. Ensuring proper design, manufacture, installation and maintenance of VRS is an important element of this objective. Appropriate life-cycle management from manufacture to maintenance is essential in ensuring systems perform in accordance with the parameters for which they were certified. Engineers involved in the design, specification, installation, supervision, certification, maintenance and road safety reviews of VRS must be aware of the current requirements and latest thinking in the area of VRS and must be able to:
  • Describe and apply mitigation measures that negate the need for VRS during the design process thus creating forgiving roadsides;
  • Classify VRS according to IS EN1317 and appreciate the technical performance parameters therein;
  • Understand the TII TD 19 design and risk assessment process for VRS;
  • Apply the requirements of the Construction Products Regulation to VRS;
  • Develop and propose bespoke solutions for retrofitting VRS on the legacy network where constraints prevent compliant designs; and
  • Identify good or bad practice in VRS installation and maintenance.
Author: Herbert Nyamakope, engineering inspector, Transport Infrastructure Ireland

Engineers working in this area may be interested in Engineers Ireland's upcoming Vehicle Restraint Systems Design Course. Dates will be published shortly on the Engineers Ireland CPD Training Calendar. To register your interest, or for further information, contact CPDTraining@EngineersIreland.ie or call (01) 665 1305.