Transport-specific thresholds
Hazard models tell you it rained 40 mm in an hour. Thresholds tell you whether that matters.
In one sentence
A transport-specific threshold is the point at which a weather or climate-related hazard begins to adversely affect a transport receptor, resulting in a reduction in service quality, operational disruption, or physical damage to an asset.
This is the component that converts a physical measurement into a transport consequence. Without it, the IMP would have flood maps and heat maps but no basis for saying a road is closed, a train is speed-restricted, or a rail is at risk of buckling.
What a threshold measures
Thresholds may represent environmental conditions such as:
- Rainfall intensity
- Flood-water level
- High or low temperature
- Wind speed
Note that temperature appears in both directions. Cold is a threshold condition too, not only heat.
Which receptors they apply to
Thresholds can apply to different road and rail receptors. The handbook lists seven, and the last one is easy to overlook:
| Receptor | Road or rail |
|---|---|
| Pavements | Road |
| Drainage systems | Both |
| Structures | Both |
| Tracks | Rail |
| Signalling systems | Rail |
| Overhead line equipment | Rail |
| Transport users | Both |
Users are receptors too
A threshold does not have to involve an asset failing. People are explicitly in scope, which is what allows the framework to capture conditions that are unsafe or intolerable for travellers even when the infrastructure itself is undamaged.
The three forms a threshold takes
Thresholds can take different forms depending on how the impact is triggered. This
classification carries through the whole chapter, and it reappears as the
Threshold Type field in the
output database.
| Form | What it represents | Typical evidence |
|---|---|---|
| Design | Environmental limits incorporated into infrastructure design standards | Industry standards |
| Operational | The point at which interventions such as speed restrictions, service restrictions or closures are implemented | Operational guidance, policy documents |
| Observation | Impacts identified through real-world experiences | Published research, recorded events |
The three are not interchangeable, and the distinction matters when results are interpreted.
A design threshold says what the asset was built to withstand. An operational threshold says what the operator actually does in practice, which is often more cautious than the design limit. An observation threshold says what has been seen to go wrong, which may not match either.
What this chapter covers
| Page | What you will find |
|---|---|
| Threshold identification | The CCRA-informed approach used to find and compile thresholds |
| Input data | The three inputs, and how exceedance is assessed |
| Output data | The threshold database and its fields |
| Further reading | Standards, policy documents and research behind the values |
Where this sits in the framework
Thresholds are the final step of the weather event component. They take hazard output from hazard models and hand a transport consequence to the impact assessment, where performance metrics quantify it.