Management processes
Four groups of analysis tools have been developed to cover the different stages of the road life.
Impact assessment
The impact on safety of transport projects or land use development should be evaluated at an early stage to avoid unintended adverse consequences, and to seek solutions for improving mobility and reducing congestion that are compatible with road safety. Before a decision is made to construct a new road or make a major change to the design or operation of an existing road, a safety impact assessment should be made [21]. This should assess impacts on the safety of surrounding roads or other transport networks, and requires network models that can show the potential casualty levels associated with different network layouts and traffic patterns.
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Examples of Safety Impact Assessment tools
In UK, the SafeNET program includes modules for building networks of nodes and links and assigning expected accident frequencies to each link and node based on their design features and usage, enabling the total number of expected casualties resulting from different choices of overall network layout, detailed local design, and alternative traffic distributions to be assessed. Data are provided for both urban and rural networks (TRL).
In Netherlands, the Explorer program provides a tool by which traffic and crash data can be plotted onto a GIS base and risk of different road sections computed. Measures can be applied to network links and the effect on risk computed. The program also includes cost model by which the costs of measures and the value of the risk reduction can be compared. Measures include non-engineering changes such as increased enforcement (SWOV).
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The European Commission has prepared guidelines requiring safety impact assessments to be carried out on all new projects for the Trans-European Road Network.
Safety audit of initial design and construction
Investigation of accidents occurring after a road has been built often indicate deficiencies in design that could have been eliminated at the design stage [1]. These deficiencies do not necessarily result from non-compliance with existing design guidelines, but more often reflect aspects of integration of design features that are not adequately covered in design manuals.
To counter this many countries have developed procedures for safety audit of designs for new road schemes. Typically, audits of major schemes might be made at three stages – preliminary design, detailed design, and pre-opening. For the largest schemes an earlier audit might be made as part of a feasibility study. For smaller schemes, the first two stages might be combined. Most countries adopting safety audit have produced detailed procedures and checklists for use by auditors [42][94][14][6][7][61]. An extensive account of the practical issues associated with auditing is provided by Proctor [71].
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Safety audits in Britain
Since 1990 audits have been required for all Highway Agency roads and by 2000 most local highway authorities have audited major schemes and many minor schemes. Audit teams consist of two staff. For the third stage, it is common for the auditor to be accompanied by a police officer and a maintenance engineer. Visits are made during both day and night time. The client has to decide whether to act on the recommendations from the audit report, but must provide an exception report justifying his decision if no action is taken.
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Safety audit in Australia and New Zealand
Safety audit has been applied for new State Highway projects in New Zealand since 1993 and subsequently by many local authorities. Audit was also adopted by Australian State highway authorities from the early 1990s and guidelines have been produced for use in both countries.
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Improving the safety design of a project at the planning and design stages can save a significant number of lives and injuries over the life cycle of the project. Comparisons of audit costs with estimates of the potential accident savings that would result from proposed modifications at the audit stage [104] [78] [7]. Transit New Zealand [94] has shown that the benefit to cost ratio of audits average typically between 10:1 and 20:1 [21].
Regular accident reduction remedial treatment
Road authorities are required to operate their roads safely. To do this it is necessary to monitor accident occurrence and to assess the scope for remedial treatment to reduce accident numbers and severity. This is most effectively done by maintaining an accident investigation and reduction team [42]. These teams are able to consider four types of treatment - black spot treatment at individual sites, route management over longer lengths of road, area treatment covering a network of roads, and mass action programmes which treat all sites at risk rather than just those where accidents have occurred in the past.
Countries with large numbers of accident black spots are likely to focus initially on treatment of individual sites. Good analysis methods are required to ensure budgets are correctly targeted – an example of Polish materials is given in Szczuraszek [83]. Low cost engineering solutions can produce high benefits at these sites [20]. As an accident reduction programme matures and accident density is reduced, the other types of treatment are likely to form a larger proportion of the programme, although individual site treatments will probably continue to be important if traffic conditions on the network change.
Accident investigation requires a combination of detailed analysis of accident data and on-site investigation.
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Example of accident analysis in France
In France the SURE approach has been developed to include driver perception of risk as a key part of assessing priorities for infrastructure improvement. Histories of high casualty rates can be used to identify sites where potential improvement is needed but similar sites will have different accident rates because drivers perceive their risk as different. Detailed analysis of accident reports are therefore needed to diagnose the causation factors. Site inspections are focussed on those road sections with high number of accidents and particular attention is paid to identifying the characteristics of sites which lead drivers to misjudge the real level of risk. Inspections aim to understand how the road functions in practice and apply remedial measures to aspects that result in incorrect function (SETRA).
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Examples of risk analysis tools to support decisions on choice of treatments
In Australia, Risk Manager has been developed to assess hazards and rank potential treatments at a site. Hazard assessment involves modifying an estimate of general crash risk at a site, by local site conditions and design factors to give a relative risk estimate to the site and an estimate of casualty severity from the crash. Changes in the risks are estimated for potential treatments, again using general estimates of effectiveness modified by site conditions. The program allows treatments to be ranked by the effectiveness at the site, and provides an audit record of the assessment made (ARRB).
In the US, Safety Analyst is being developed to help identify and manage a system wide programme of site-specific improvements involving physical modifications to the highway system. It includes tools for network screening, diagnosis, countermeasure selection, economic appraisal, priority ranking and evaluation of implemented measures (FWHA).
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To understand the effectiveness of accident improvement programmes it is important both to monitor the overall trend in accidents on the network, and also to record the effectiveness of the individual measures introduced. Knowledge that a particular measure has only had limited effectiveness at a particular type of site should lead to more efficient use of resources.
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UK MOLASSES database of results of implementation of engineering schemes
Highway authorities in UK are encouraged to provide data to a central database on the effectiveness of low cost engineering measures implemented in their programmes. Information requested includes simple details of the site, type of measure implemented, costs, and accident numbers 3 years before and after the treatment. No attempt is made to describe site conditions in detail or to correct the difference in accident numbers for other factors varying between the time intervals. The results therefore are intended to give an indication of the average effects of treatments at sites which have generally been chosen because of their high risk; comparing data over time also gives some indication of whether treatment effects estimated on this basis are reducing. The response from authorities in providing such data is variable and thus the extent to which the results give a true picture across all sites treated is unclear [32].
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Audits of existing roads
Where historical accident data are sparse or assessment of the road safety risk can also be made through on-road inspections. These may focus on one specific aspect of road design or attempt to provide overall assessments of risk.
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Safety audit of existing roads in France
Machu [54] describes a survey of roadside treatment over 2500 kms of road inspecting primarily the occurrence of aggressive roadside objects near to the road. In isolation however this information provides only a limited basis on which to assess the value of remedial action. A sample of urban roads was also inspected [96] and this work has been extended to assess ways of providing more forgiving urban roadside environments [97].
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Audit of existing roads in New Zealand
Transfund New Zealand is developing the concept of safety audit of existing roads, looking for recurring patterns of deficiencies across the network, as a means of assessing how well road controlling authorities are performing.
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Road Protection Score ratings in EuroRAP
The European Road Assessment Programme is developing a more comprehensive assessment of the extent to which road design protects road users from serious injury. This can be set alongside historical data on accident occurrence on these roads to indicate the scope for various treatments to improve protection. Similar programmes in Australia and North America are seeking to incorporate accident occurrence ratings within the overall assessment [51].
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