#25 Barrier End Treatments

Following our previous posts on road safety barriers and motorcycle underrun protection, the current brief looks at terminal end treatments for semi-rigid (e.g., W-beam) barriers. Also referred to as crash cushions or impact attenuators, terminals reduce impact forces by absorbing kinetic energy, and may redirect a vehicle post-impact, thereby reducing crash severity. According to the Austroads Guide to Road Design Part 6, barrier terminals are required to meet the requirements in the Australian and New Zealand Standard, AS/NZS 3845.1; 2015. Products meeting the Standard generally align with specifications in the Manual for Assessing Safety Hardware (MASH) published by the American Association of State Highway and Transportation Officials.

Effectiveness

The effectiveness of terminals varies according to many factors including but not limited to terminal design characteristics and properties, location and placement, vehicle mass, trajectory and impact speeds. Also, published evaluations tend to consider the guardrail as a system rather than the terminal specifically or in isolation. They are, however, generally highly effective for reducing the severity of crashes involving light vehicles and are an essential component of most barrier treatments.

Classification and Specification

Austroads describes several classifications of terminals and crash cushions, including that they are either:

• “redirective” or “non-redirective”. Most current terminals are redirective.

• “gating” or “non-gating” based on their ability to redirect vehicles that impact along the side of the terminal

• “flared” or “tangent” (parallel) designs

• “energy-absorbing” or “non-energy-absorbing

MASH test levels (TL) refer to the maximum impact at which a barrier (including terminal) system can be expected to be effective. The level is assigned after multiple controlled tests of the hardware using different vehicle types. Barrier and terminal systems in Australia usually meet TL2 or TL3 specification. The higher-rated TL3 is tested effectively for light vehicles (including most SUVs and utility vehicles) impacting at speeds up to 100km/h and an angle of 25 degrees. TL2 meets the same “containment” specifications for speeds up to 70km/h. Terminals rated at these levels are not expected to effectively contain or redirect heavy vehicles. More information may be sourced through the Austroads Safety Barrier Assessment Panel (ASBAP).