Road Safety Manual
A manual for practitioners and decision makers
on implementing safe system infrastructure

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11.2 Project-level and Network-level Approaches

Developing network-wide assessments to guide programme development is an essential part of a comprehensive road safety programme for a road authority. This should be based on a good understanding of system-wide risks and issues. Project-level identification of risk that is consistent with the higher level network-focused programme approach is also required. Both are discussed in Assessing Potential Risks And Identifying Issues. In a similar manner, network-level approaches can be taken to address this identified risk through effective infrastructure interventions. In order to address system wide issues, wide scale implementation of treatments can be undertaken.

Perhaps the best documented example of this approach is the implementation of the 2+1 road design scheme in Sweden, as described in the case study below.

Box 11.1: Case Study – 2+1 road design in Sweden

The problem: It was recognised that the rural road network with a 13 m cross-section had a very poor safety performance when compared to other road types.

The solution: In 1998, the Swedish Road Administration (SRA) commenced an innovative road safety programme with the aim of providing safe 13 m cross-section roads in a cost-effective manner.

The 13 m roads featured a single lane in each direction with wide shoulders. The 2+1 treatment involved narrowing the shoulders to 1 m, and providing two lanes in one direction and one lane in the other, with a narrow median and flexible (‘cable’ or ‘wire rope’) barrier in the median. Bergh and Petersson (2010) concluded that modern flexible safety barriers, like the ones installed on Swedish collision-free roads, are the single most important factor affecting the performance of roadside safety. Flexible barriers absorb the impact energy, and in most cases avoid the serious injury often associated with head-on collisions (Larsson, Candappa & Corben, 2003). As part of the design, the middle lane changed direction every few kilometres.

The outcome: As of January 2008, approximately 1,800 kilometres of collision-free roads have been opened. Programme evaluation revealed a 76% reduction in fatalities, with safety performance that was equivalent to a motorway. In addition, after analysis of the 2+1 road design, it has been predicted that the reduction in fatalities and serious injuries is 65–70% for motorcyclists. This design approach is now used in many countries in Europe, with trials also undertaken in other parts of the world.

An example of the 2+1 road design is shown below.

An example of 2+1 road

 

 

As well as network-level implementation of specific treatments (such as the 2+1 design identified above), there is also scope to develop frameworks at that provide guidance on application of treatment types. As an example, a framework has been developed in New Zealand for guiding safety investment decisions based on the level of collective and/or individual crash risk. Figure 11.2 below draws on this approach and illustrates how collective and individual risk could be used to influence about cost effective outcomes. Individual risk refers to road safety risk as it applies to any one road user. It is often expressed as the chance of any given road user being involved in a crash (often in crashes per vehicle kilometre travelled, which takes into account traffic volume. Other metrics are also available). Individual risk is a useful measure for assessing the quality (in safety terms) of road infrastructure. Collective risk refers to the total expected crash outcome for all vehicles (e.g. crashes per kilometre), and is heavily influenced by traffic volume. Using information on collective and individual/personal risk the treatment types can be categorised into four groups:

  • Safe System transformation;
  • safer corridors;
  • safety management;
  • safety maintenance.

Figure 11.2 Framework for treatment selection on high risk rural roads - Source: Adapted from Durdin & Janssen (2012).

Roads with high traffic volumes have a high expected number of severe crashes; and those that include road engineering features which are substandard for the function, are likely to score highly in both collective and individual risk areas (the red area in Figure 11.2). Substantial investment into road safety treatments on such roads would often be justified via Safe System Transformation works, e.g. a major upgrade; provision of an alternative, higher quality route; freeway style interchanges, etc. Further examples of these higher cost, but highly effective treatments can be found in Effective Safe System Interventions in Intervention Option and Selection.

Roads that experience intermediate collective and individual risk outcomes fall in the Safer Corridors or Safety Management categories (orange and yellow areas in Figure 11.2). For example, highways in rural areas with moderate traffic volumes, some localised and scattered severe crashes, and compromised road design, may fall in the Safe Corridors area. The most effective treatment approach may be via corridor-wide improvements using a mix of high- and low cost solutions (e.g. safety barrier installations, line-marking, intersection upgrades, etc.).

Safety Management ideas may apply to roads with lower traffic volumes, more scattered severe crashes (e.g. local streets and roads) and consistently inadequate road standards. The best economic return on safety would be via network-wide and/or corridor-based application of low cost treatments, e.g. speed limit revisions, line-marking treatments, or targeted asset management (e.g. pavement resurfacing with associated safety treatments, including shoulder sealing). This group also includes roads with high collective severe crash risk due to high traffic volumes, but with a good overall road safety standard (e.g. urban motorways). The most cost-effective actions may be based on targeted systemic changes, e.g. managed freeways techniques and infrastructure supported enforcement.

Roads with low collective and individual risk (green area in Figure 11.2) are most likely candidates for Safety Maintenance activities. Safety Maintenance often involves incremental and systemic changes such as through road management (e.g. skid resistance management), improvements to signs and line-markings, and other good maintenance practices.

Figure Figure 11.2 shows that as collective and individual risk increases, more extensive treatments are likely to be applicable. As risk progresses to higher categories, benefits from applying treatment options from the lower categories should also be considered.

Although developed and implemented in an HIC, the approach outlined is equally useful in LMICs, particularly in the upgrade of existing road infrastructure. The approach may form an effective way of helping to prioritise road safety activity.

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