Dimensions of road safety problems and their measurement

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Dimensions of road safety prob-lems and their
measurement

• Rune Elvik, Institute of Transport Economics,
  Oslo, Norway

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Levels and objectives of road safety analyses

• International, national, regional or local level
• An applied objective to understand in order to
  prevent
• Analysis of road safety problems at the national
  level for the purpose of developing a road safety
  programme

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Some important questions

• What is a road safety problem? What do we mean by
  it?
• What is a rational analysis of road safety
  problems?
• What information is needed to perform a rational
  analysis of road safety problems?
• What are the basic dimensions of road safety
  problems? How can they be assessed?

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A road safety problem

• Any factor that contributes to the occurrence of
  accidents or the severity of injuries

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Objective of analysis

• To identify those problems that make the
  greatest contribution to accidents or injuries
  and that are amenable to treatment

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Elements of a rational analysis

• A taxonomy categorising potential problems
• A method for assessing the size or importance of
  problems
• A concept of the amenability of problems to
  treatment
• Access to relevant data

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Taxonomies 1- The Haddon Matrix
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Taxonomies 2- Vision Zero

• Inadequate system design (not based on human
  tolerance for injury)
• Road user vulnerability
• Unsafe road user behaviour (violations)
• Environmental risks
• Inadequate trauma care

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Taxonomies 3 basic risk factors

• Kinetic energy
• Friction
• Visibility
• Compatibility
• Complexity
• Predictability

• Road user rationality
• Road user vulnerability
• System forgiveness

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Dimensions of problems

• Magnitude (size of contribution)
• Severity
• Externality
• Complexity
• Inequity
• Spatial distribution
• Dynamics
• Perceived urgency
• Amenability to treatment

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Measuring dimensions

• Magnitude is indicated by population attributable
  risk
• Severity is indicated by gradient in relative
  risk with respect to injury severity
• Externality is indicated by the imposition of
  risk by one group of road users upon another
• Complexity is indicated by the relative
  contribution of specific risk factors to overall
  risk

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Measuring dimensions

• Inequity is indicated by degree of inequality in
  risk and lack of proportionality between risk and
  benefits of transport
• Spatial distribution is indicated by the degree
  of geographic concentration of accidents
• Dynamics is indicated by trend over time in the
  size of a problem

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Measuring dimensions

• Perceived urgency is indicated by level of
  support for interventions designed to solve a
  problem
• Amenability to treatment is a function of
• Complexity
• Perceived urgency
• Knowledge of effective measures

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Data that are ideally needed

• Exposure to risk (amount of travel) for each
  group of road users
• Exposure to specific risk factors (related to a
  chosen taxonomy)
• Road user behaviour with respect to, for example
  speed, seat belt wearing, etc
• Injured road users by severity

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Data needed - continued

• Surveys of perceptions of road safety problems
• A catalogue of potentially effective safety
  measures
• Preferably time series of data for all items

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The distribution of risk
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Three sources of risk

• Internal risk involving members of a specific
  group only
• Risk imposed by other groups injuries to
  members of group 1 in accidents with counterparts
  2, N
• Risk to other groups (external) injuries to
  members of groups 2, N in accidents involving
  group 1

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An example (excerpt)
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An example of complexity
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Limitations of analysis

• Biased and incomplete accident reporting
• Availability of data (distractions, fatigue)
• Choosing the right level of analysis
• Possibility of statistical analysis (violations
  of expectations)
• Treatment of correlations between problems

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Some observations

• A comprehensive analysis of road safety problems
  is needed to develop effective road safety
  programmes
• The analysis of road safety problems is complex,
  as these problems are multi-dimensional
• Analysis should start by choosing a taxonomy to
  help classify problems

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Further observations

• A list of potential problems in each category of
  the taxonomy should be made
• The dimensions of problems to be emphasised in
  analysis should be chosen
• Create profiles of problems
• Determine promising targets for intervention
  based on these profiles