29 April 2015

Technical Talk: Road Safety Barrier Length of Need

The length of need (LoN) is a common term used in relation to road safety barriers. But what does it actually mean?

LoN for a road safety barrier installation is the length of barrier required to shield a roadside hazard and is determined during the barrier design phase.

The barrier system within the LoN must be able to redirect the impacting vehicle towards the road away from the hazard. The overall LoN of the barrier is the sum of the leading LoN, the length of the hazard and the trailing LoN. Note that the trailing LoN is only included if there is two way traffic with no median barrier - meaning that a vehicle from the opposing direction could impact the hazard.

The beginning and end of the LoN in the design may also be referred to as the PoN or the point of redirection. The point of need (PoN) is the point where the barrier system becomes redirective. A leading end terminal will have a PoN at or downstream (in the direction of traffic) of where any redirection will occur. On a two way road for the trailing end terminal, downstream refers to the opposing traffic’s direction of travel.

An end terminal PoN is determined through crash testing and is specified in the supplier’s product information. CSP’s X-350 End Terminal has a PoN at post 1, instead of post 3 (as for other terminals). As a result the X-350 is fully re-directive, providing a greater length of protective barrier and reducing the cost of installation.

The Austroads Guide to Road Design Part 6: Roadside Design, Safety and Barriers provides both technical guidance and worked examples for determining length of need. To view the Austroads Guide to Road design, click here.

The New Zealand Transport Agency uses the Austroads guide to road design as the primary reference guideline for New Zealand’s road network. Links to the Austroads guides and supplementary guidance are available on the Geometric design page of the NZ Transport Agency website, click here.