Threshold identification
Within the DARe framework, transport-specific thresholds are identified using a CCRA-informed threshold identification and compilation approach for UK road and rail transport systems.
The method works from an authoritative starting point rather than a blank page. The transport operators have already assessed their own climate risks; this approach takes that assessment and asks what measurable value sits behind each risk.
Step 1: Select the risks worth investigating
The National Highways (NH) and Network Rail (NR) Climate Change Risk Assessments (CCRAs) are used to identify the transport receptors and climate-related risks requiring threshold investigation.
Two filters narrow the field:
| Filter | Effect |
|---|---|
| Severity | The initial assessment prioritises risks classified as major or severe |
| Relevance | Risks not directly associated with transport-system operation are subsequently screened from the threshold assessment |
Why filter at all
A full CCRA covers a great many risks, and most will never bind on a transport simulation. Prioritising major and severe risks concentrates effort where threshold exceedance would actually change an outcome, and the second filter removes risks that are real but sit outside transport operations.
Step 2: Find the threshold values
Threshold values associated with the selected risks are then identified from four kinds of evidence:
- Relevant industry standards
- Operational guidance
- Policy and industry documents
- Published research
These sources are documented in detail on the input data page.
Step 3: Record the value, or the formula
Not every threshold reduces to a single number, and the approach accommodates both cases.
| Situation | How it is recorded |
|---|---|
| A single measurable threshold can be identified, such as a specific temperature, wind speed, rainfall intensity or flood-water level | The value is recorded directly |
| Impacts depend on several interacting variables | A formula-based or locally derived threshold may be used instead |
Why formula-based thresholds exist
Some failure modes genuinely have no single trigger value. Rail buckling risk, for instance, depends on more than air temperature alone.
Forcing such a case into a single number would either be wrong or would have to be so conservative as to be useless, so the approach keeps the relationship rather than flattening it.
Step 4: Compile into a database
The identified thresholds are compiled into a consistent database structure, so that they can be:
- Linked back to the original CCRA risks
- Integrated into DARe modelling, analysis and case studies
Consistency is the point of this step. Thresholds arrive from standards documents, operational guidance and academic papers, each with its own conventions. Normalising them into one structure is what makes them usable as a model input rather than a reading list.
The resulting structure is described in full on the output data page.