RISK ACCEPTABILITY CRITERIA FOR RAILWAYS

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RISK ACCEPTABILITY CRITERIA FOR RAILWAYS

• Odd Nordland
• SINTEF, Trondheim, Norway
• odd.nordland_at_sintef.no
• Frank Renpenning
• SIEMENS Transportation Systems, Brunswick,
  Germany
• frank.renpenning_at_siemens.com

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RISK ACCEPTABILITY CRITERIA FOR RAILWAYS

• Introduction
• Railways involve risks that have to be reduced to
  an acceptable level
• CENELEC Standards
• propose principles for determining acceptability
• do not specify acceptance criteria
• Railway administrations define criteria for risk
  acceptance
• Need a comprehensible definition of "risk"
• Then the criteria can be determined

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RISK ACCEPTABILITY CRITERIA FOR RAILWAYS

• Risk measurement
• Risk is defined differently in different
  standards
• risk probability x consequences
• Which consequences?
• For railways harm to passengers
• Collective risk vs. individual risk
• Collective risk number of victims per year
• absolute values
• will increase with traffic increase
• Individual risk accident probability per person
  and year
• relative values
• more practical to use

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RISK ACCEPTABILITY CRITERIA FOR RAILWAYS

• Risk acceptance principles
• ALARP (UK)
• As Low As Reasonably Practicable
• reduce unacceptable risks to acceptable level
• effort ("price") must be reasonable and
  practicable
• GAMAB (FR)
• Globalement Au Moins Aussi Bon (globally at least
  as good)
• allows for trade-offs
• similar to ALARP
• MEM (DE)
• Minimum Endogenous Mortality
• objective principle, but risk is subjective
• controversial, difficult to apply

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RISK ACCEPTABILITY CRITERIA FOR RAILWAYS

• Risk Acceptance Criteria (1)
• Norway
• start with tolerable hazard rates (THR) for SIL4
  according to standards
• total hazard depends on equipment types and
  quantities, deployment profiles, traffic density
  etc.
• apply ALARP with THR as boundary between
  unacceptable and acceptable
• Sweden
• not more than 1 hazardous event per 100 years in
  the railway network
• must new equipment compensate for old equipment?
• for calculations, assume identical equipment
  everywhere
• effectively an application of GAMAB

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RISK ACCEPTABILITY CRITERIA FOR RAILWAYS

• Risk Acceptance Criteria (2)
• Germany
• Apply "accepted rules of engineering practice" or
• if not, show that the same safety level has been
  achieved
• Effectively an application of GAMAB
• "German" MEM is controversial
• UK
• ALARP is used
• relates acceptability to necessary effort
• effort is expressed with "Value of Preventing a
  Fatality"
• "ALARP is a philosophy for continuous
  improvement"
• Must be supplemented with "good practice"

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RISK ACCEPTABILITY CRITERIA FOR RAILWAYS

• Conclusions
• There is no set of generally accepted risk
  acceptance principles
• Harmonisation is necessary for interoperability
• difficult, because risk acceptance is a political
  question
• Standards propose principles
• leave the details to the legislative bodies
• MEM is not generally accepted
• GAMAB appears to be most widespread
• results are not uniform
• ALARP has greatest potential to bring improvements